Copying starts fast because the data first lands in a fast memory buffer, either your computer's RAM cache or the SSD's high-speed cache, and the speed reading shows that buffer filling up. Once the buffer is full, the transfer drops to the drive's true sustained write speed, which is slower. This is normal behavior, and it is most obvious with USB drives, hard drives, and large or many small files.
Why this happens
- Write caching in RAM. The operating system accepts data into fast memory first and reports that quick speed, then writes it to the slower drive in the background. When the cache fills, the display drops to the drive's real speed.
- SSD cache tiers. Many SSDs keep a small fast cache that behaves like high-speed memory. Large transfers fill that cache, after which writes fall back to the drive's slower main storage.
- Sustained versus burst speed. Hard drives and USB flash drives can accept a short burst quickly but have a lower steady write speed they can hold for a long transfer.
- Many small files. Each file carries overhead for creating and indexing it, so thousands of tiny files copy far slower than one large file of the same total size.
- Heat. Some SSDs slow down when they get warm during a long transfer, a normal protection called thermal throttling.
Is this normal?
Yes. Starting fast and settling to a lower steady speed is normal for almost every drive. The high number at the start is the cache filling, and the lower number is the drive's actual sustained write speed.
Does it cause any problems?
No. Nothing is wrong and no data is at risk. The slowdown is just the transfer showing its real speed after the fast cache is used up. The steady rate you settle into is the honest measure of how fast the drive can write.
How to fix or check it
- Treat the sustained speed as the real one. Ignore the brief peak at the start; the steady rate is what the drive can actually maintain.
- Use a faster port and cable. Plug USB drives into a USB 3.0 or newer port and use a cable rated for it, since an old port caps the whole transfer.
- Combine small files. Zip many tiny files into one archive before copying to avoid the per-file overhead, then extract at the destination.
- Check both drives. The transfer runs at the speed of the slower of the source and destination, so an old external drive will limit even a fast internal one.
- Keep the drive cool. Give a hot SSD a moment to cool during very large transfers, or improve airflow if it throttles often.
- Close other disk activity such as backups or downloads that compete for the same drive.
Frequently asked questions
Is the slowdown a sign of a failing drive?
Usually no. Dropping from a fast start to a steady lower speed is normal caching behavior. A drive may be failing if speeds are extremely slow throughout, the transfer stalls repeatedly, or you see read or write errors.
Why do small files copy so much slower than large ones?
Every file adds overhead for creating it and updating the filesystem index. One large file is a single continuous write, while thousands of small files mean thousands of separate operations, which takes much longer even at the same total size.
How do I find my drive's real transfer speed?
Watch the speed after the first several seconds, once it settles into a steady number. That sustained figure, not the brief peak at the start, reflects what the drive can genuinely write over a long transfer.