You start copying a folder to a USB stick and the meter reads a healthy 300 MB/s. Thirty seconds later it has collapsed to 5 MB/s, or it freezes at 99% and just sits there while you wonder if the drive died. The short answer to why file transfers slow down: it is almost never a fault. It is the predictable behavior of cheap flash memory, your operating system’s cache, and the filesystem you formatted the drive with.
The good news: once you understand what is actually happening, the slowdown stops feeling like a glitch and starts looking like physics you can plan around. Here is what is going on inside that little stick, and what you can change.
Most flash drives and SD cards write to a small, fast SLC cache first, then drop to their much slower native TLC or QLC speed once that cache fills, which is the cliff you see mid-transfer. Thousands of small files crawl because each one carries filesystem overhead, and a copy stuck at 99% is usually your OS flushing its write buffer. Cheap drives also thermally throttle during long writes. Use exFAT, copy fewer-but-bigger files, and pick a metal-bodied drive.
The SLC cache: fast at first, then a cliff
Modern flash stores multiple bits per memory cell. TLC packs three bits and QLC packs four, which is cheap and dense but slow to write. To hide that, drives reserve a slice of memory that runs in fast single-bit SLC mode and use it as a write cache. Your first few gigabytes land in that cache at high speed, which is the snappy number you see at the start.
When the cache fills, the drive has to write directly to the slow native cells and simultaneously drain the cache into them. Speed falls off a cliff. On a well-made TLC drive the floor can hold in the hundreds of MB/s; on budget QLC drives it often drops below 100 MB/s, and the worst sticks sink into single digits, which is exactly the “dropped to 5 MB/s” experience. Worse, on drives that use a dynamic cache, the SLC area shrinks as the drive fills up, so a nearly full drive gives you far less fast headroom before the cliff hits. This is the same mechanism that affects SSDs, but flash drives and SD cards have tiny caches and weak controllers, so the drop is brutal and obvious.
Thousands of small files are the real killer
Copying one 20 GB video file is easy: the system opens it once and streams a long, contiguous block. Copying 20 GB spread across 100,000 photos and documents is a different job entirely. For every single file the OS has to locate it, open it, write its data, update the filesystem’s allocation tables and metadata, then close it. Each of those steps carries a fixed latency that barely matters once but adds up to crippling overhead across tens of thousands of files.
That is why your transfer can read “45 minutes remaining” for a pile of tiny files but breeze through a single large one of the same size. If you regularly move many small files, zip them into one archive first and copy that. The drive gets one long sequential write instead of a storm of tiny random ones, and the difference is often several times faster.
Why it gets stuck at 99%
The progress bar is a guess, and the last stretch is where the guess unravels. To feel fast, your OS reports a file as “copied” the moment it lands in RAM (the write buffer), not when it is safely on the drive. At the end of the job that buffer still has to be physically flushed to the slow flash, plus the filesystem has to finalize all the directory and allocation-table entries. So the bar parks at 99% or says “Finishing” while real writing continues in the background.
On Windows you can influence this. In Device Manager, a removable drive’s Policies tab offers “Quick removal” (little buffering, safe to yank, and the default since Windows 10 version 1809) versus “Better performance” (more buffering, faster, but you must use Safely Remove Hardware or risk corruption). The takeaway: do not pull the drive when the bar hits 100% or the window vanishes. Wait for the eject confirmation, because that is when the flush actually completes.
Heat: the throttle you cannot see
A long, sustained write keeps the controller and NAND chips working hard with no idle time to cool off. When the temperature crosses a safety threshold, the controller deliberately slows the drive to protect the chips from damage. This is thermal throttling, and it stacks on top of the cache cliff, so a drive can get slow for two reasons at once.
Casing matters more than you would think. A metal-bodied drive acts as a heatsink, spreading heat across its surface and letting the chips keep running. A plastic drive feels cool in your hand precisely because plastic insulates, trapping heat inside where the chips sit. For long transfers, a slightly warm metal drive is often doing better than a cool plastic one. If a drive becomes too hot to comfortably hold, give it a break.
Filesystem choice: exFAT, FAT32, or NTFS
How you format the drive sets both compatibility and how much per-file overhead you pay. The headline trap is FAT32’s hard 4 GB limit per file, a leftover of its 32-bit length field, so a single HD movie or disk image simply will not copy. exFAT lifts that ceiling far past any consumer file and works across Windows, macOS, modern consoles, and cameras, which makes it the best all-round choice for big files on removable media. NTFS is fast and robust but is really a Windows filesystem, with read-only or clunky support elsewhere.
| Filesystem | Max file size / best use |
|---|---|
| FAT32 | 4 GB minus 1 byte per file; widest device compatibility, fails on large files |
| exFAT | Effectively unlimited (volumes up to 128 PB); best cross-device choice for big files and large drives |
| NTFS | 256 TB per file on modern Windows; great on Windows, poor support on Mac/cameras/consoles |
Do this
Format big drives as exFAT, zip thousands of small files into one archive before copying, buy a metal-bodied drive, and wait for the eject confirmation before unplugging.
Not that
Do not assume the start speed is sustained, do not yank the drive at 99%, and do not run long transfers on a tiny plastic stick that is already hot.
So is your drive broken?
Almost certainly not. A fast start that decays to a crawl, a freeze at 99%, and a warm casing are all normal behavior for consumer flash. The fixes are mostly about working with the hardware: use exFAT, batch small files, buy a drive with a real metal body and a known-good controller, and be patient at the finish line. If you are managing space on the source device too, our guide to freeing up storage without deleting anything pairs well with this.
Frequently asked questions
Why did my USB stick start at 300 MB/s then drop to 5 MB/s?
The drive wrote your first few gigabytes into a small, fast SLC cache, then exhausted it and fell back to the slow native TLC or QLC memory underneath. Budget QLC drives can drop below 100 MB/s, and the cheapest controllers sink into single digits. A nearly full drive makes this worse because a dynamic cache shrinks as free space drops.
Why is copying many small files so much slower than one big file?
Every file carries fixed overhead: the system has to find it, open it, write its data, update the filesystem’s metadata and allocation tables, then close it. That cost is trivial once but punishing across tens of thousands of files. Zipping them into a single archive first turns thousands of small writes into one fast sequential write.
Is it safe to unplug when the transfer is stuck at 99%?
No. The progress bar reaches 99% when files are in your computer’s RAM buffer, not when they are physically on the drive. The last stretch is the OS flushing that buffer and finalizing filesystem entries. Always wait for the Safely Remove or eject confirmation, or you risk corrupting files.
Should I format my flash drive as exFAT, FAT32, or NTFS?
Use exFAT for most drives, especially for files over 4 GB, since FAT32 cannot store a single file larger than 4 GB and exFAT works across Windows, macOS, cameras, and consoles. Choose NTFS only if the drive will live almost entirely on Windows machines, because Macs and many devices read it poorly.
Does a hot flash drive really slow down on purpose?
Yes. During long, sustained writes the controller can cross a temperature threshold and deliberately throttle speed to protect the NAND chips from heat damage. Metal-bodied drives shed heat better and hold speed longer, while plastic drives trap heat inside even though they feel cool to the touch.