The macOS installer is designed to prevent you from installing macOS on a Time Machine backup disk. It's a good idea to keep your startup disk and backup disk separate, in case your startup disk becomes unusable.
If you no longer want to use your Time Machine backup disk for backups, you can either erase the entire backup disk, or follow these steps to erase just the backups on the disk. You can then install macOS on the disk.
These steps delete the backed up files stored on the backup disk. If you need that data, install macOS on a different drive, or copy your backup data to another drive first.
- Choose Apple menu () > System Preferences, then click Time Machine. Turn off Time Machine or deselect ”Back Up Automatically.”
- Locate the Backups.backupdb folder on your Time Machine backup drive. This folder is normally at the top level of your backup drive. You can use Spotlight to help find it.
- Move the Backups.backupdb folder to the Trash. Enter an administrator name and password, if prompted.
- Empty the Trash to delete the Backups.backupdb folder.
You can now install macOS on the drive. When you're ready to turn Time Machine back on, make sure you have another drive you can use to store your backups.