The operating system and the drive itself each have a level of cache to defer writes to the disk and combine them with others to make the process more efficient with less random seeking. USB keys are usually configured with writing cache disabled and are slower, particularly when copying many small files.

When the drive is instructed to turn off, all outstanding data is written to the disk. The heads also get retracted from the surface of the disks and parked on a dedicated ramp. The drive tries to do this in case power is suddenly removed, but won't be able to orderly write out any buffered data.