My external disk made a clicking noise. What should I do? I have a USB2+Firewire external disk that made a clicking noise, and Mac OS X immediately unmounted it and put up a dialog box saying that a peripheral is misbehaving. The disk made a clicking noise several times in a short interval, and each time it did, the dialog box on screen would flash. So I unplugged it, waited for a few seconds, and plugged it in to another Mac, and this time it worked perfectly. I copied all 800 GB of data out of the drive over some 9 hours, and it worked perfectly and at its usual speed of 30 - 40 MB/second.

Now that all my data is out of the disk, I repaired the filesystem using Disk Utility. What else should I do? Should I run some kind of disk scan tool that checks the entire disk for bad blocks? Can you suggest a tool for the Mac? Since the disk has a HFS+ partition (and a FAT32 partition), I think I should run a disk repair tool on Mac rather than Linux. Is that correct?

I don't have SMART data since it's an external disk, and I don't want to break open the enclosure.

Thanks.
Clicking noises are generally signs of immanent disk failure. Backing up your important data on that drive is a good idea, and you should do that first thing. 

Most HDD manufacturers have tools that will perform thorough checks on your disks health and come in different formats (most require an image to burned to a CD/DVD and then boot to it).

Do you know what the model of the external drive or the hard disk it houses?