USB stick or external drive copying slowly, or the speed drops after a few seconds? Why, and what actually helps
The progress bar flies for a few seconds, then the speed falls off a cliff and the estimate jumps from two minutes to an hour. Or the drive was never fast at all. There's usually one clear reason.
Which slowdown do you have?
| What you see | Most likely | What helps |
|---|---|---|
| Fast for a few seconds or a few GB, then drops sharply | The computer's memory filled first, then the drive's fast cache (often called SLC cache). After that you get its real sustained speed. | Nothing is broken. For big regular copies, buy a drive whose reviews quote sustained write speed, not just a peak. |
| Never goes above about 30-40 MB/s | Running at USB 2 (480 Mbit/s): an old port, a USB 2 hub, a USB 2 cable, or a stick that is USB 2 itself. | Check the speed (below), plug straight into the computer, use the cable that came with the drive. |
| A folder of photos or code crawls, big videos copy fine | Every small file needs its own create, write and close step, plus antivirus checks. | Zip the folder first and copy one big file, or use a tool like robocopy or rsync. |
| Speed swings between full and near zero | The stick is getting hot and slowing itself down, or a hard drive is shuffling data on the fly. | Let it cool, copy in batches, or use a proper external SSD for large jobs. |
| Goes to 0 and hangs for a while | A failing drive retrying bad blocks, or a cable dropping out. | Try another cable and port. If it keeps happening, copy what matters off it now. |
| Slow only on one Windows PC | Windows is set to Quick removal, so it writes everything straight through. | Normal and safer. Better performance is faster but you must eject every time. |
What speed to expect
| Connection or drive | Label | Real copy speed, roughly |
|---|---|---|
| USB 2 | 480 Mbit/s | 25-40 MB/s |
| USB 3.2 Gen 1 (also sold as USB 3.0, 5 Gbps) | 5 Gbit/s | up to about 400 MB/s, if the drive can keep up |
| USB 3.2 Gen 2 (10 Gbps) | 10 Gbit/s | up to about 1,000 MB/s |
| Cheap USB stick, writing | often "up to 100 MB/s" (that's reading) | 5-30 MB/s once its cache is full |
| Portable hard drive | 5 Gbit/s port | 80-150 MB/s on big files, slower near the end of the disk |
| Portable SSD | 10 Gbit/s port | 400-1,000 MB/s, some drop after tens of GB |
The quick sum: time is size divided by speed. 50 GB at 20 MB/s is 50,000 / 20 = 2,500 seconds, about 42 minutes. At 400 MB/s it's about 2 minutes. Labels use bits (Gbit/s) and copy dialogs use bytes (MB/s), and there are 8 bits in a byte, so 5 Gbit/s is at most 625 MB/s before any overhead. The unit converter does the conversion.
Check what speed the drive is connected at
- Mac: hold Option, open the Apple menu, choose System Information, then USB. Click the drive: Speed: Up to 480 Mb/s means USB 2; 5 Gb/s or 10 Gb/s means USB 3.
- Windows: a stick on a slow port often triggers "This device can perform faster". For a definite answer, Microsoft's free USBView tool shows each device's speed (High-Speed is USB 2, SuperSpeed is USB 3).
- Linux: run
lsusb -t. 480M is USB 2, 5000M or 10000M is USB 3.
Blue or red plastic inside a USB-A port, or "SS" next to it, usually means USB 3. Many cheap USB-C hubs and some phone charging cables carry only USB 2 data, which is the most common hidden bottleneck.
Other things that slow a copy
- Antivirus scanning each file as it lands. Copy a test file with it paused briefly to see the difference.
- A nearly full drive. SSDs and sticks get slower when they're almost full because they have less room to shuffle data.
- Copying from a slow source. A copy can only be as fast as the slower drive, and a phone over a cable is often the slow one.
- Mac to an NTFS drive doesn't copy at all: see external drive read-only on a Mac.
- Format makes little difference to speed. exFAT, NTFS and APFS all perform similarly on big files. FAT32 is fine too, but it can't hold files over 4 GB.
Related
SSD slower than advertised · Fake USB stick test · Check if a slow drive is failing · Windows says "This device can perform faster" · Drive clicking or beeping · USB stick or external drive not showing up · Mac won't eject an external drive · USB-C cable types · Which format for a USB stick for a TV, car or console · Video size calculator · Interactive: is it the stick, the port or the paperwork? (Wirebook)
Questions
Why does my file transfer start fast and then slow down?
The computer's memory and the drive's small fast cache fill first. After that you see the drive's real sustained write speed, which on cheap sticks can be 5-30 MB/s. It's normal, not a fault.
How do I know if my USB drive is running at USB 2 speed?
On a Mac, check System Information, USB: Up to 480 Mb/s means USB 2. On Linux, lsusb -t shows 480M. On Windows, USBView shows High-Speed for USB 2 and SuperSpeed for USB 3. A copy that never beats about 40 MB/s is another clue.
Why is copying lots of small files so slow?
Each file needs its own set of steps and often an antivirus check, so thousands of small files take far longer than one big file of the same total size. Zip them into one file first.
Does formatting a USB stick as exFAT or NTFS make it faster?
Not noticeably for big files. Pick the format for compatibility, and buy a faster drive if speed matters.
How long should it take to copy 100 GB to a USB drive?
Divide size by speed: at 30 MB/s about 56 minutes, at 100 MB/s about 17 minutes, at 400 MB/s about 4 minutes.
Published · updated .