Apple has made Time Machine backup disks read-only to protect the integrity of the backups. The whole disk is reserved for Time Machine only. If you want to write other files to a Time Machine backup drive, you must add another APFS volume to it. Here's how to do it:
- Open Time Machine Settings/Preferences.
- Select the APFS external hard drive and click the remove button ( - ).
- Click "Forget Destination."
- Open Disk Utility.
- Select your APFS container and click the add button ( + ) above "Volume."
- Name it and click Add.
Now, you can save files to the Time Machine drives.
