If you copy files to a USB flash drive and remove it right after cp finishes, data may still be in the kernel write cache.

That means copy command is done, but device writes are not fully done yet.

The safe approach is simple:

  1. Copy files
  2. Force data to disk
  3. Unmount device
  4. Remove flash drive

Basic safe flow

cp -r ./my-folder /mnt/usb/
sync
umount /mnt/usb

Linux vs Windows write cache behavior

On Linux, if you run umount without sync first, umount may take a while. That is normal: Linux flushes pending cached writes before it completes unmount.

On Windows, removable drives are usually configured for quick removal, so writes are flushed more aggressively. Because of that, eject is often instant after copy is fully completed, and people often remove drives without manual eject.

Copy with stronger write guarantees

If you want to reduce cache effects during writing, use tools that flush writes:

dd if=file.txt of=/mnt/usb/file.txt bs=4M oflag=dsync

rsync --fsync file.txt /mnt/usb/

oflag=dsync and --fsync make writes more explicit and safer for removable media workflows.

Seeing sync progress

If you want visible sync progress (instead of waiting blindly), use this helper script.

It wraps sync and shows progress/ETA based on dirty pages, which is useful before unplugging a drive.

thanks @tsalkenov for help