Time Machine - Change External hard drive preference

I have an external hard drive I want to use with Time Machine on my MacBook Pro. I had selected "Don't Use" at one point. How do I change that preference? It's not displaying that drive as an option.
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Plug in the hard drive. Go to Apple > System preferences> Time Machine. You can click on "Select Disk" on there.
Make sure that the drive is formatted for Mac before using with Time Machine for better results.

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BTW I also like Time Machine Editor http://timesoftware.free.fr/timemachineeditor/ because it allows you to set the times that it backs up, so you don't get system slowness every hour.
... but try just letting it do its thing first for a week or so. You probably won't notice any slowdowns and you will have a very reliable, up to date backup.
ealebrunDeveloperAuthor Commented:
The problem is that I would need to reformat the disk for Mac. I ended up buying a new external.
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