The OS X Installer is designed to prevent you from installing OS X on a Time Machine backup disk. It's a good idea to keep the files on your startup disk on a separate drive from your backup, in case the drive becomes unusable.
If you previously used a drive to store Time Machine backups and you no longer want to use it as a backup disk, you can use the steps below to install OS X on the drive. If you no longer need any of the data on the drive, you can erase the volume instead.
Performing these steps deletes backed up files stored on the backup drive you select. If you need the data on this backup drive, install OS X on a different drive, or copy your backup data to another drive before using these steps.
- Choose System Preferences from the Apple Menu.
- Click Time Machine in the System Preferences window, then turn off Time Machine.
- In the Finder, locate the Backups.backupdb folder. This file is normally at the top level of the volume where your backups are stored. You can use Spotlight to help you find this folder.
- Move the Backups.backupdb folder to the Trash. Enter an administrator name and password if prompted.
- Empty the Trash to remove the Backups.backupdb from the volume.
You can now install OS X on this drive. When you're ready to turn Time Machine back on, make sure you have another drive you can use to store your backups.