Hard Drive Icon Changes Color

The icon for rmy exernal drive changes flashes from dull gray to a nice color every so often. Only for a few seconds at a time. This is on the OSx 6 desktop. I only use this external drive for backups. It does it even when I'm not accessing the drive for backup purposes. Why?

When I open Finder, the left panel shows this external drive in orange color and my internal drive shows in solid gray. None of them flash in different colors.

Will reboot - tha seems to cure all kinds of flakie things but still have to wonder: "Why"?

Thanks if you can help.