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A large file copy can begin at an SSD’s advertised speed, then slow sharply once the drive’s temporary write buffer fills or cannot free space quickly enough. Many SSDs use NAND flash in single-level-cell (SLC) mode as a fast “lane” for incoming data, then reorganize that data into the drive’s native format. The slowdown is consistent with the buffer running out, but it is not proof: the exact SSD, available space, workload, source device, firmware, and connection all affect the result.
What makes an SSD slow down during a large copy?
Many SSDs temporarily write incoming data to NAND flash configured in SLC mode. SLC stores one bit per cell, which lets the drive accept writes quickly. The drive later re-stores that data in its native NAND format, such as TLC or QLC. SK hynix describes this sequence as writing quickly in SLC mode and then re-storing data in its original format to improve write performance (SK hynix, April 8, 2026).
The fast buffer is finite. During a long write, it can fill, or the drive may be unable to make enough room in it. Incoming data may then be written at a lower sustained rate. The size of the buffer, when it becomes unavailable, and the speed after that point vary by drive; there is no universal file size or post-buffer speed.
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A manufacturer’s peak sequential-write figure describes a best-case performance specification, not necessarily the speed a drive can maintain through a very large write. Samsung’s 2020 870 QVO announcement, for example, lists up to 530 MB/s sequential write and says the drive uses a variable SLC buffer. That peak figure does not establish the 870 QVO’s post-buffer rate. Samsung also cautions that performance can vary with firmware, hardware, and configuration (Samsung Electronics Canada, June 30, 2020).
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The 870 QVO illustrates why a buffer figure must be tied to a specific model and capacity. Samsung listed a maximum TurboWrite buffer of up to 42 GB for the 1 TB model and up to 78 GB for the 2 TB, 4 TB, and 8 TB models. These were manufacturer figures for that product line, and the buffer adjusts within usable disk space; they are not standard SSD buffer sizes or a guarantee of how much data any other drive can accept at peak speed.
SLC buffering, DRAM, and NAND type are different things
SLC write buffering is not the same as DRAM cache
An SLC buffer uses NAND flash temporarily configured to store one bit per cell. DRAM is separate memory. Samsung’s 980 announcement, for instance, describes the drive as DRAM-less with Host Memory Buffer support while also listing TurboWrite. Those are distinct features, not different names for the same cache (Samsung Electronics America, March 9, 2021).
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TLC or QLC alone does not determine a drive’s speed
NAND labels describe how many bits each cell stores: SLC stores one, TLC stores three, and QLC stores four, according to SK hynix. NAND types differ in performance and endurance characteristics, but a cell label by itself cannot tell you a drive’s exact sustained write speed. Controller, firmware, capacity, buffer design, and workload matter too (SK hynix, April 8, 2026; Kingston Technology, SSD guide).
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A copy is limited by more than the destination drive. The source device must supply data, and the computer’s interface or external connection can cap throughput. Benchmark results can also vary with the test tool and system configuration. A fast SSD cannot overcome a slower link or source device (Kingston Technology; Samsung Semiconductor USA FAQ).
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Use this diagnostic sequence when a copy starts quickly and then drops:
- Identify the destination drive. Confirm which device is receiving the data and find its exact model and capacity. A slowdown during a write is different from a slower read from the source.
- Look at the transfer pattern. A high initial rate followed by a lower rate during a long write is consistent with a filled or unavailable SLC buffer, but does not confirm that cause by itself.
- Check the other limits. Consider whether the source can keep up and whether the host interface or external connection is restricting the transfer.
- Compare like with like. Find sustained sequential-write measurements for the exact SSD, especially results taken after its write buffer is exhausted. Peak specifications alone cannot explain the later rate.
Samsung identifies Magician as a drive-management and performance resource, but the cited information does not establish that it reports remaining SLC-buffer capacity. A utility or setting should not be assumed to prevent the drive’s normal post-buffer rate change.
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What to compare when choosing an SSD for large writes
If you regularly write large files, prioritize evidence about the rate the exact drive can sustain after its buffer fills. Check compatibility and endurance as well as the headline speed.
- Post-buffer sustained writes: Look for measurements on the precise model and capacity, with the test conditions stated.
- Capacity and usable space: Buffer size can depend on model and capacity, and some designs adjust it within usable disk space.
- Interface compatibility: Confirm the drive fits the computer and that the connection will not be the limiting factor.
- Endurance and warranty: Compare the manufacturer’s endurance rating, commonly expressed as TBW (terabytes written), and warranty terms.
- NAND, controller, and firmware: Treat NAND type as one design factor, not a substitute for sustained-write evidence.
Samsung’s 2021 980 announcement gives another model-specific example: it lists up to 160 GB for the TurboWrite region. That figure is not a general cache-size rule, and it does not establish a universal post-buffer speed (Samsung Electronics America, March 9, 2021). The available model examples do not provide a current, independent cross-drive benchmark or a basis for ranking today’s SSDs.
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