USB cache (or write caching) is when data is first stored in temporary memory before being written permanently to the flash drive. This helps speed up small writes, but when copying a large file, the cache can fill up.

Once the cache is full, the system has to wait while the drive finishes writing data to flash. That’s why a transfer may look like it pauses or slows down, and then resumes at a different speed — it’s the cache emptying and refilling in the background.

Always use “Safely Remove Hardware” to be sure all cached data has been written before unplugging the drive.