How to disable Time Machine for an external hard drive My wife has a MacBook Pro and two external hard drives; both of which currently contain a Time Machine backup of her MacBook Pro hard drive.

Now she wants to "turn off" Time Machine for one of her two hard drives, so that she can backup the MacBook Pro hard drive and the other external hard drive onto it.

In addition to the Time Machine backup, there are also additional files on the hard drive, so she cannot reformat it unless she first copies the files she wants to keep to somewhere else.

Is it possible to tell OS X "Do not recognise this external hard drive for use with Time Machine any more", so that it can be backed up using Time Machine? Currently, the external drive she wants to back up shows up in the "Exclude these items from backups"-list and cannot be removed, likely because it has been used as a Time Machine drive in the past.

How can she make it so that the one external hard drive (Drive A), can be backed up to the other external hard drive (Drive B), without reformatting it.

I appreciate any help with this.
Go to Time Machine preferences, Ctrl-click on the drive you want to disable, you get a fold out menu with the option to disable the drive.