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You can combine older hard drives into a RAID array, but there is no universal recipe. The correct layout depends on your operating system or storage platform, the number and actual capacities of the drives, the level of fault tolerance you need, and whether you can keep a separate backup. Creating an array or pool can erase the selected disks, so copy and verify valuable data before you begin.
Use this guide to plan the array, test the disks, choose between Linux mdadm and TrueNAS/ZFS, and carry out the build without confusing redundancy with backup. The platform’s current documentation must supply the final commands or interface steps for your specific release.
What a RAID array does—and what it does not do
RAID combines multiple physical disks into one logical storage arrangement. Depending on the level, it may stripe data for capacity and throughput, mirror data for redundancy, or calculate parity so a failed member can be reconstructed. The operating system normally sees the result as one block device, pool, or dataset rather than a collection of independent volumes.
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Old disks are especially unsuitable for a “build it and forget it” approach. TrueNAS documentation states: “Hard drives and solid-state drives (SSDs) have a finite lifetime and can fail unexpectedly.” A SMART result is evidence about a disk’s current condition; it is not a guarantee that the disk will survive the next day, a rebuild, or years of service.
Choose the platform before choosing the layout
Linux software RAID with mdadm
mdadm is Linux’s software RAID management utility. The Linux mdadm manual documents multiple RAID levels, assembly, monitoring, replacement, and recovery operations. The exact procedure depends on your distribution, boot arrangement, filesystem, encryption, and whether the array will hold the operating system or only data. Use the manual and your distribution’s current documentation for the final command sequence rather than copying a command intended for a different machine.
An mdadm array normally sits below a filesystem, and you may add LVM or encryption above it. That layering affects how you expand, recover, and identify devices, so record the intended stack before creating anything.
TrueNAS and ZFS
TrueNAS presents disks as a ZFS pool made from one or more vdevs. A vdev can be a mirror, RAIDZ group, or stripe. Pool-level redundancy is determined by the vdevs: adding a nonredundant vdev can make the pool vulnerable even if another vdev is mirrored or RAIDZ. TrueNAS replacement, resilvering, and expansion behavior is version-sensitive; select the stable release you are running and follow that release’s documentation and interface labels.
Do not translate an mdadm layout directly into a TrueNAS design. ZFS has its own terminology, pool rules, checksumming, scrubs, and vdev expansion behavior.
Decide how much capacity and fault tolerance you need
For unequal disks, plan with the smallest usable member capacity, not the rounded capacities printed on retail packaging. mdadm ordinarily sizes components from the smallest member. TrueNAS limits a vdev to the usable capacity of its smallest attached device. A larger disk can therefore leave part of its space unused until a supported expansion or replacement strategy makes that space available.
| Common layout | Typical minimum | Usable-capacity rule with equal-size disks | Failure behavior | Important limitation |
|---|---|---|---|---|
| RAID 0 stripe | 2 disks | Approximately the sum of all members | No disk failure tolerance | Loss of one member loses the array; TrueNAS requires recreating a failed stripe and restoring from backup. |
| Two-way mirror (RAID 1) | 2 disks | Approximately the capacity of one member | One member can fail while the mirror remains available | A second failure before repair can destroy the mirror. |
| Single-parity RAID (commonly RAID 5 or RAIDZ1) | 3 disks | Approximately total capacity minus one member | One member failure is tolerated | The array has reduced redundancy until the failed disk is replaced and reconstruction completes. |
| Dual-parity RAID (commonly RAID 6 or RAIDZ2) | 4 disks | Approximately total capacity minus two members | Two member failures are tolerated | More capacity is consumed by parity and writes are implementation-dependent. |
| Mirrored stripes (commonly RAID 10) | 4 disks | Approximately half the total capacity | Usually one failure per mirror pair; the exact tolerated combinations depend on which members fail | It is not safe to assume that any two failed disks are tolerated. |
These are planning rules, not a promise that every controller or ZFS release exposes identical options. Check the layout’s documented drive-count requirements and failure model before clicking Create. If the data is replaceable and you have a current backup, a stripe may be acceptable for temporary working space; it is a poor choice for the only copy of personal or business data.
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Inventory the old drives
Make a written inventory before connecting disks to the array host. Record:
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- Manufacturer and model
- Serial number or another stable identifier
- Capacity reported by the operating system
- SATA, SAS, USB, or other interface
- Existing partitions and filesystems
- Where the disk is physically connected
- Any data that still needs to be copied
Stable identifiers matter because device names such as /dev/sda can change when disks are moved or rebooted. Match the identifier and reported capacity in the platform before selecting a member. Photographing cable positions and labeling drive bays can prevent a destructive selection error.
Confirm that the motherboard, HBA, controller, or adapter exposes every disk individually. A USB enclosure, hardware RAID card, port multiplier, or controller operating in a hidden RAID mode can prevent the operating system from seeing the drives as independent members. A controller or adapter is only useful when the host lacks suitable ports or visibility; it does not make an unhealthy disk reliable.
Back up and verify before creating anything
Array creation, pool creation, disk replacement, and some partitioning operations can overwrite metadata and user data. Copy valuable files to independent storage first. “Independent” means a separate device or system, not another directory on one of the disks about to be reused.
- Copy the files to the backup destination.
- Open representative files and check important directories.
- Use a checksum or the backup application’s verification feature when available.
- Keep the backup disconnected or otherwise protected while you create the array.
- Only then mark the old disks as disposable members.
Do not rely on a degraded array as the only place to hold the backup. If a disk already contains the only copy of a file, stop and preserve that disk until the copy has been verified elsewhere.
Test each disk before assembly
Read SMART information
Use the SMART facilities provided by Linux, TrueNAS, or the disk’s controller to inspect reallocated sectors, pending sectors, uncorrectable errors, temperatures, power-on information, and recent self-test results. Attribute an alarming result to the individual drive, not to the array as a whole. A disk that reports errors, repeatedly drops from the bus, or cannot complete a self-test should not be trusted as a new member.
Run a suitable self-test
A short test is a quick screening step. A long or extended HDD test reads substantially more of the media and can take a long time; schedule it when the disk is not needed for other work. Archived TrueNAS CORE guidance describes starting a long HDD self-test and warns that the age and condition of used drives may be uncertain. Let the test finish, review the result, and keep the report with your inventory.
Passing a self-test does not certify a disk for years of service. Treat a marginal result, an unknown history, or a disk that becomes unusually slow or hot as a reason to replace it before building a critical array.
Check the mechanics and connections
- Listen for repeated clicking, spin-up retries, or unusual grinding.
- Check that SATA or power connectors are firmly seated.
- Inspect event logs for link resets, timeouts, and disappearing devices.
- Allow adequate cooling; several old disks in one enclosure can raise temperatures.
Check media type and physical compatibility
Do not assume that all old disks behave alike. TrueNAS hardware guidance distinguishes conventional magnetic recording (CMR) and shingled magnetic recording (SMR) media and advises checking the manufacturer to identify the technology. Mixed models may have different sustained-write behavior, firmware, cache sizes, and error-recovery characteristics. The documentation does not establish universal compatibility for every mixture.
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Compare actual device-reported capacity, sector format, interface, and power requirements. A replacement that is marketed as the same nominal size can be slightly smaller in reported sectors and be rejected. mdadm documents that a replacement may fail this check, while TrueNAS requires a replacement disk with capacity equal to or greater than the failed member’s requirement.
Build the array or pool only after a final identity check
- Shut down workloads and unmount any filesystems that belong to the disks.
- Recheck each selected disk’s stable identifier, serial number, and capacity against your inventory.
- Confirm the intended level, expected usable capacity, and tolerated failures.
- Confirm that the backup is readable and separate.
- Use the current official procedure for your selected platform and release.
- Wait for initial initialization, synchronization, or parity construction to complete before treating the storage as protected.
- Create the filesystem, dataset, or share only after the underlying array or pool reports a healthy online state.
- Record the member order, layout, serial numbers, and recovery instructions in a location separate from the server.
There is no safe universal command block for “old hard drives.” The same-looking command can erase the wrong disk on a different machine. In Linux, follow the mdadm manual and distribution documentation for assembly, persistence across reboot, filesystem creation, and monitoring. In TrueNAS, use the pool-creation workflow for your stable release and confirm the resulting vdev layout before adding data.
Linux mdadm: decisions and checks
Choose the array level and device size
mdadm normally uses the smallest component to determine the common array size. If one disk is smaller, the extra space on larger members is not automatically available to the array. Leave room for a replacement: a drive that barely matches the original’s rounded capacity may still be smaller in sectors and rejected.
Place the filesystem and other layers deliberately
Decide whether the array will contain a filesystem directly or sit beneath LVM, encryption, or another volume layer. Document the order. During recovery, you must assemble the md device first and then activate the layers above it; mixing an old filesystem signature or an old array superblock into the new design can create confusing results.
Enable monitoring and persistence
Configure the distribution’s mdadm monitoring and alerting facilities, ensure the array is assembled automatically at boot, and test that an alert reaches an address you read. A silent degraded array can continue serving files while its remaining members age, turning a recoverable failure into data loss.
TrueNAS/ZFS: pools, vdevs, and resilvering
Understand the vdev boundary
ZFS distributes data and redundancy within each vdev. A pool made from multiple vdevs depends on every vdev remaining available, so do not add a stripe vdev to a pool that is expected to protect important data. A vdev’s usable capacity is limited by its smallest attached device.
Use the documented replacement workflow
TrueNAS warns that a disk is wiped during its replacement workflow. Select the failed member by serial number and bay, choose a replacement with equal or greater required capacity, and confirm every destructive prompt. The system then resilvers the pool. Wait for the resilver to finish and the pool to return to a healthy state before replacing another member or starting a major workload.
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TrueNAS documentation describes several different expansion actions:
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- Adding a new vdev: adds another independent group to the pool and changes the pool’s aggregate capacity and failure dependencies.
- Replacing every member with larger disks: can make additional space available after the replacement process and platform conditions are satisfied.
- Extending a RAIDZ vdev: current documentation describes rewriting data across the new configuration while maintaining the RAIDZ fault-tolerance level. The pool should be healthy before starting. Older data blocks retain their original data-to-parity ratio until they are rewritten.
These behaviors and interface labels can change between TrueNAS releases. Verify them against the stable version installed on your system rather than relying on a screenshot or instruction written for a development build.
What happens when a disk fails
First identify the failed member from the platform’s alert, serial number, and physical bay. Do not pull a disk solely because a device name looks different after reboot. If the disk is intermittently disconnecting, capture logs and confirm the cable, power, and controller before declaring the media dead.
Once the member is confirmed, obtain a replacement that meets the platform’s capacity requirement. In TrueNAS, the replacement must be equal to or larger than the failed member’s required capacity. Follow the platform’s replacement operation; do not initialize the new disk in a way that hides it from the array manager.
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Reconstruction or resilvering reads the surviving members and writes the replacement. It can take a long time, particularly when a pool contains a large amount of data. During this period, the array has less redundancy and may be slower. Avoid unnecessary heavy workloads, maintain cooling, and monitor errors. If another member fails before completion, the result depends on the layout and which failures it can tolerate.
Expansion is not the same as replacement
Replacing one disk with a larger disk does not automatically make every RAID implementation larger. Some layouts require replacing all members before additional capacity becomes usable. Adding a vdev is a different operation from enlarging an existing vdev and can change the pool’s failure dependencies. RAIDZ extension is a separate, version-specific ZFS feature.
Plan expansion before the first build. If you expect to add disks later, check whether your chosen platform can grow the selected layout, whether the new disks must match the existing width, and whether the process rewrites data. Keep enough free space and a verified backup before beginning any expansion.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Monitor the array after it is online
- Enable SMART polling and scheduled self-tests.
- Schedule filesystem or ZFS integrity checks where supported.
- Configure alerts for degraded state, failed self-tests, temperature, checksum errors, and unavailable devices.
- Review alerts rather than dismissing them as old-drive noise.
- Keep a tested replacement drive or a realistic procurement plan for the array’s member size.
- Check backups by restoring files periodically.
Redundancy is most useful when a failed disk is replaced promptly. A degraded mirror or parity set can survive the original failure but has fewer defenses against the next one. Never postpone a replacement merely because the shares are still accessible.
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One disk is missing from the creation screen
Power down and inspect the cable, power connector, controller mode, and enclosure. Confirm that the operating system sees the disk individually. Do not substitute a different disk until you have reconciled the serial number with your inventory.
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The platform rejects a replacement as too small
Compare the exact reported sector capacity, not the rounded label. A disk sold under the same terabyte rating can have fewer usable sectors. Use a drive that meets the documented equal-or-greater requirement.
The array is degraded immediately after creation
Check event logs, SMART results, cabling, and cooling. A degraded state can indicate a bad disk, a link problem, or an interrupted initialization. Do not repeatedly force the member online without understanding the error; that can worsen corruption.
You selected the wrong disk
Stop writes immediately. If the original data still exists on another independent copy, recreate the arrangement only after confirming every identifier. If no backup exists, avoid additional “repair” commands and seek data-recovery advice; rebuilding over the disk can destroy remaining metadata and files.
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A rebuild or resilver appears stuck
Check progress, device errors, temperatures, and controller resets. A slow process can be normal with large or heavily used disks, but repeated read errors, disappearing members, or rising uncorrectable counts require investigation. Do not replace another disk until the platform reports completion or gives a documented recovery path.
When using old drives is reasonable
Reusing disks can make sense for disposable scratch space, a laboratory system, media that already exists elsewhere, or a learning project where downtime is acceptable. It is harder to justify for irreplaceable photos, financial records, production databases, or any workload for which you cannot buy a compatible replacement quickly.
Make the decision from the complete system, not the drive count alone. A smaller redundant array with healthy, matched members and tested backups is safer than a larger array assembled from unknown disks. If you cannot maintain monitoring, backups, cooling, and timely replacements, do not put critical data on the array.
A final preflight checklist
- Every disk is identified by stable identity, serial number, capacity, and physical location.
- Valuable data has a separate, verified backup.
- SMART information and suitable self-tests have been reviewed.
- The host exposes each disk individually and provides adequate cooling and power.
- CMR/SMR status and interface compatibility have been checked with the manufacturer where necessary.
- The chosen platform and stable version support the intended layout.
- Usable capacity was calculated from the smallest member, with parity or mirroring accounted for.
- The tolerated failure combinations are understood.
- Monitoring, alerts, scheduled tests, and a replacement plan are ready.
- Expansion and recovery steps are documented before data is written.
If any item is false, pause the build. RAID creation is quick compared with reconstructing data after a preventable mistake.
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