Why your USB 3.0 drive transfers files slowly

Your USB 3.0 drive says it can transfer data quickly, but copying files takes much longer than expected. The cause could be the drive, the port or adapter, or the files you are copying.

Start by measuring the drive itself, then compare that result with a real file copy. The USB & SD Speed Test measures read and write speed so you can narrow down where the slowdown is.

Test the USB drive

1. Connect the drive directly to a USB 3 port on your computer. Avoid a hub, dock, or extension cable for the first test.
2. Open the USB & SD Speed Test and follow the on-screen instructions to test the drive. If the browser asks for access, select the USB drive you want to check.
3. Note both the read and write results, including the units. Read speed is how quickly the computer gets data from the drive. Write speed is how quickly it saves data to the drive.
4. Repeat the test in another USB port, then copy one large file and a folder with many small files. This helps show whether the port or the type of files changes the result.

Keep other copies and downloads paused while testing. If you compare different ports, use the same drive and the same test so the results are easier to interpret.

What the result means

Read and write speeds are both low

The drive may be connected through a slower port, hub, or adapter, or the drive itself may have modest performance. A USB 3.0 connection is rated at 5 Gbps, but that is a signaling rate, not a promise of 625 MB/s file copies. Encoding, protocol overhead, and the drive's flash memory reduce actual speed.

Check the connection first, then compare the drive on another USB 3 port or computer. A low result on several computers points more strongly to the drive.

Read speed is good, but write speed is low

Many flash drives read data much faster than they write it. Write speed can also drop during a long copy after a small fast buffer fills, or when the drive is nearly full. Compare the test result with a single large file copy that lasts long enough for the speed to settle.

The test looks fast, but copying files is slow

The USB connection may be fine. A folder containing thousands of small files can take much longer than one large file of the same total size because the computer must handle each file separately. The source drive can also be the bottleneck, especially if you are copying from an older hard drive or a network location.

Check whether the slowdown happens in both directions. If reading from the USB drive is fast but saving to it is slow, focus on write performance. If both directions are slow only during normal copying, check the source, destination, and file sizes.

The speed is far below expectations and files have errors

Unusually low speeds, files that become corrupted, or a drive that reports more space than it can really store can point to a failing or counterfeit drive. If you suspect the advertised capacity is false, see how to check a USB drive's real size. Back up anything you can still read before testing further.

How to fix a slow USB 3.0 transfer

1. Use a direct USB 3 port. Connect the drive to the computer instead of a hub or dock. Look for a port marked with the USB trident and an SS label, or check the computer's manual. A USB-C connector does not automatically mean the port supports USB 3 speeds.
2. Check the connection speed. On macOS, open Apple menu › System Settings › General › About › System Report › USB and look for the drive's listed speed. On Windows 11, right-click Start, open Device Manager › Universal Serial Bus controllers, and check that your computer has a USB 3.x controller. Device Manager confirms the controller is present, but may not show the speed negotiated by the individual drive.
3. Compare one large file with many small ones. Copy a single file that is at least a few gigabytes, if you have one, and compare it with a folder of small files. If the large file moves much faster, the number and size of files are a major part of the delay.
4. Check the source drive. Copy the same large file from a different internal drive or computer. A slow source cannot provide data quickly enough for the USB drive to reach its full write speed.
5. Free space on the USB drive. If it is nearly full, move or remove files you no longer need and test again. Some drives also slow down during long writes, so use the settled speed rather than the fastest moment at the start.
6. Try another port, reader, or computer. If you are using an SD card in a USB reader, test another reader too. A loose connection or a slow card reader can limit the card even when the card itself is faster.

When the USB drive may be faulty

If the drive remains slow on multiple computers and USB 3 ports, and its write speed is far below what its manufacturer specifies, the drive may be worn out or defective. Repeated disconnects, read errors, or corrupted files are stronger warning signs than a low speed result alone.

Copy important files elsewhere while the drive is still readable. If it is under warranty, contact the manufacturer or retailer about a replacement. Avoid trusting it with the only copy of important files.

Why the number can change during a copy

File transfers often start quickly and then slow down. Some flash drives use a small, fast memory area to absorb incoming data. After that area fills, the drive writes directly to slower flash memory. A short transfer may finish before the slowdown appears, while a large transfer shows the drive's sustained speed.

Speed units can also be confusing. MB/s means megabytes per second, while Mbps means megabits per second. There are eight bits in a byte, so a result shown in Mbps will have a numerically larger value than the same speed shown in MB/s.

The USB & SD Speed Test runs in your browser, and your personal files stay on your device.