A big file copy can begin at high speed and then drop sharply because many SSDs use part of their NAND flash as a temporary fast-write buffer. When that buffer fills—or the drive cannot make more room in it—the SSD may write at a lower sustained rate. That pattern is consistent with cache exhaustion, but it does not prove it: the exact drive, free space, source device, workload and connection all affect transfer speed.
What the SSD “fast lane” is
NAND flash can store data at different bit densities. An SSD with TLC or QLC NAND can temporarily use some flash in single-level-cell (SLC) mode to accept incoming writes quickly, then re-store that data in its original format. SK hynix describes the process as “first writing data quickly in SLC mode and then re-storing it in the original format” (SK hynix, April 8, 2026).
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The buffer is temporary and finite. During a sufficiently long write, it can fill or become unavailable; the drive may then write at a lower sustained rate. The point at which that happens and the resulting speed depend on the SSD model and workload. There is no universal file size or post-buffer speed that applies to every drive.
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Why the advertised speed can mislead on a big copy
A manufacturer’s peak sequential-write figure describes a particular performance condition, not necessarily the rate the drive can maintain after its buffer is used. For example, Samsung’s 2020 870 QVO announcement lists up to 530 MB/s sequential write and describes its Intelligent TurboWrite feature as using a variable SLC buffer. The peak figure does not establish the 870 QVO’s post-buffer rate, and Samsung cautions that performance varies with firmware, hardware and configuration (Samsung Electronics Canada, June 30, 2020).
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The 870 QVO illustrates why buffer size must be tied to the exact model and capacity. Samsung listed a maximum variable TurboWrite buffer of up to 42 GB on the 1 TB model and up to 78 GB on the 2 TB, 4 TB and 8 TB models. Those were manufacturer figures for that product line; Samsung said the buffer adjusts within usable disk space. They are not general SSD cache sizes, nor do they establish how quickly a particular drive will write after the buffer is used.
Another model can have a different design: Samsung’s 2021 announcement for the 980 cited a TurboWrite region of up to 160 GB. That is a model-specific maximum, not a value to apply to other SSDs (Samsung Electronics America, March 9, 2021).
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SLC caching is not the same as DRAM cache
SLC caching refers to using NAND flash temporarily in single-level-cell mode. It is distinct from DRAM, which is separate memory used in some SSD designs. Samsung’s 980 announcement describes that drive as DRAM-less with Host Memory Buffer support while also describing TurboWrite; these are separate features. A drive being DRAM-less does not, by itself, mean it lacks an SLC write buffer.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsNAND type matters, but it cannot predict speed by itself
SLC stores one bit per cell, TLC stores three and QLC stores four, according to SK hynix’s 2026 definitions (SK hynix, April 8, 2026). NAND types differ in performance and endurance characteristics, but cell type alone cannot tell you an SSD’s exact speed. Controller, firmware, capacity, buffer behavior and workload also matter (Kingston Technology, SSD NAND flash memory guide).
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How to check whether the buffer is the likely bottleneck
- Identify the destination drive. Confirm which disk is receiving the copied data and find its exact model and capacity. Use the manufacturer’s specifications or management resources for that model; do not infer behavior from the brand or NAND type alone.
- Look at the transfer pattern. A graph that starts fast and then falls during a long write is consistent with the buffer filling, but it is not proof. A short copy may finish before the drive reaches that behavior.
- Check the rest of the path. The source device must be able to supply data, and the host interface or external connection can cap throughput. Kingston explains that host limits affect transfer rates (Kingston Technology).
- Compare against sustained-write evidence for the exact SSD. Look for measurements taken after the write buffer is exhausted, not just peak sequential-write specifications. Results depend on test tools, hardware and configuration, as Samsung notes in its 870 QVO announcement.
Samsung identifies Magician as a drive-management and performance resource, but the cited information does not establish that it reports remaining SLC cache. Nor does the available evidence establish that a cache setting, defragmentation or a particular utility can prevent the inherent post-buffer rate change. Samsung Semiconductor’s FAQ discusses write cache and performance checking (Samsung Semiconductor USA FAQ).
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to compare when choosing an SSD for large writes
- Post-buffer sustained writes: Use tests for the exact model and capacity, and check how the test was run.
- Capacity and buffer behavior: Buffer size can vary by model and capacity, and available space can matter to a variable buffer.
- Compatibility: Confirm the drive’s interface matches the computer or enclosure. A faster SSD cannot overcome a slower host connection.
- Endurance and warranty: Compare the drive’s endurance rating, commonly expressed as TBW (terabytes written), as well as its warranty. NAND types have different endurance characteristics, but the rating for the specific model is what matters.
- Price: Consider cost alongside sustained performance and compatibility rather than treating a peak-speed label as the whole comparison.
The available model examples are historical product announcements, not a current cross-model benchmark or ranking. They do not establish today’s inventory, pricing or comparative value. A peak sequential-write figure alone is not enough to choose an SSD for repeated large-file transfers.
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