If the issue that deleting files doesn't free up space occurs on two new external USB flash drives on Mac, the culprit could not be the drive, you should make sure you have emptied the Trash on Mac to thoroughly delete the items. Just right-click the Mac Trash on the Dock and choose Empty Trash, then check whether the occupied space is released on your connected USB flash drive.
In addition, the same issue could also happen to the Mac's internal drive. When deleting files doesn't free up space on Mac, you can try these solutions:
- Close the document or other files before deleting
- Restart Mac
- Disable Time Machine and disconnect your backup disk
- Deleting files in Safe Mode
- Fix the startup disk errors in macOS Recovery Mode
