Are you the administrator of this Mac device? If not, you need to use the admin account to change the permissions to the USB drives.
Here's how to change permissions on a Mac:
- Right-click the USB drive.
- Select Get Info.
- Scroll down and click the arrow next to Sharing & Permissions to expand it.
- Click the lock icon and the bottom, and enter the admin account to unlock for changes.
- Click the privilege next to your user name and change "Read-only" to "Read & Write".
If you still can't write to the USB drives, it could be caused by your system errors. You can reset NVRAM or update your Mac until the issue is fixed.