Turn off the ‘Use disk for Time Machine?’ warning

In OS X 10.5, each time you connect an external drive for the first time, the system will thoughtfully ask if you’d like to use that drive for your Time Machine backup. The first time you see this, you think “Wow, that’s a nice touch, Apple! Always thinking about what’s best for me!”

But if you’re like me, FireWire and USB drives come and go through your workspace like dust in the wind. As such, by the 10th time you see that dialog, you’re instead thinking “I’ve said no nine times in a row now, I think you’d be able to figure out how I’m going to answer that question by now!” I’ve got four amazingly powerful CPU cores and 4GB of RAM at my disposal, and still, that’s apparently not enough power to hazard a guess at my answer? But I digress…

Thankfully, there’s a way to disable this alert. It requires one command in Terminal and the first version below assumes you’ve already set up Time Machine on one drive (if you haven’t, the required preferences file won’t yet exist). Open Terminal (in Applications -> Utilities), and enter this command:

defaults write com.apple.TimeMachine DoNotOfferNewDisksForBackup -bool YES

From now on, when you connect external drives, OS X will not ask if you’d like to use them with Time Machine. To restore the default behavior, repeat the above command, but replace YES with NO. If you haven’t set up Time Machine, but would still like to get rid of the warning, you can try this command:

defaults write /Library/Preferences/com.apple.TimeMachine DoNotOfferNewDisksForBackup -bool YES

That will write the preference in the global file, which is what OS X should be using when it can’t find the local preferences file. Note, however, that I haven’t tested this version myself, as all my machines are using Time Machine. Nothing bad will happen if it doesn’t work, however. As before, you can undo it by changing YES to NO.

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