The legendary Iomega
click of death is a horribly disturbing sound that is indicative of drive failure. Within a short time after hearing the click of death, you can be pretty certain that your Iomega drive is going to stop working altogether (unless you're merely using a corrupted disk that was damaged by another drive than your own).
A common misconception is that the click of death is itself a problem. In fact, this isn't true at all. The clicking you hear your drive making is merely the sound of the read/write heads being retracted from the cartridge into the drive and then being reinserted. The drive does this automatically whenever it is having trouble reading, writing, or locating data on a cartridge, since the removal and reinsertion of the heads recalibrates the positioning mechanism, scrubs the heads to remove oxide deposits, and eliminates electrostatic charge buildup.
Thus, by the time you hear your drive clicking, the damage has already been done. The data on your disk has been corrupted in some way, which is what causes the repetetive clicking. It's important to note that the click of death seems like a disease -- when you take a disk from one drive that's experiencing the click of death and put it into another drive, the second drive will begin clicking as well. As stated above, the clicking is a normal reaction to the corrupted data on the disk, so chances are the second drive is in perfect working order. It's the first drive you should worry about, since it likely corrupted the data to begin with.
There are many factors that can cause a drive to corrupt the data on a disk. Bad external power supplies, loose power connectors, excessive oxide buildup on the heads, magnetic interference, rough handling or mistreatment of the drive, and manufacturing defects.
There are no preventative measures that can be taken to reliably ensure that your disks remain uncorrupted. Nearly all Iomega drives, it seems, will eventually fail. But never fear; as a result of a class action lawsuit and millions of user complaints, Iomega tends to be very quick about replacing damaged drives. I've had several Iomega drives that succumbed to the click of death, and after contacting Iomega I usually received a replacement within a few days.
As for rescuing data corrupted by failing drives, many people have had luck using a program called SpinRite, which you can get more information on at http://grc.com/spinrite.htm.