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Like many Mac users I have an external Firewire disk hooked up to my Mac to be used by Time Machine. This works very well, backup-wise.

The problem is that very often when I use a Mac application and try to open a file, the file selection dialogue window hangs until the external disk has spun up. I never ever want to open a file on the external disk. Sometimes this happens even when I just want to save a file I already saved (i.e. type something and press meta-s).

Is there anything I can do about this?

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If an acceptable solution to you is having the drive not spin down in the first place, you can set than in System Preferences -> Energy Saver. Then you won't have to worry about it spinning up and halting the system.

I don't think you can change the behavior because it's not feasible for the system to guess when you'll need the drive available and when you won't. There might be SOME circumstances when you can be sure, but not in every case. For example: A dialog box comes up asking me where I want to store something. The system has no way to know that you'll NEVER want to store it on the sleeping disk, so it has to make the resource available (and refresh the system cache). Anytime the system needs to know what resources are available, it has to wake up sleeping devices. The solution is to not let your peripherals sleep when the computer is in use. Or else get used to waiting for them to wake up. You're probably not saving much power. Wear and tear is greater on disk drives that spin up and down than those that stay spinning. The system can't read your mind and anticipate exactly when it will need to wake something up.

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I think this only applies to internal disks. External disks seem to fall asleep whenever they want rather than when the system allows it. (Incidentally, it would be feasible for the system to wait until I select the disk in a dialogue window before starting it up.) – Andrew J. Brehm Jun 18 '10 at 15:06

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