I've had this problem a lot. I have a 500GB external hard drive. 1 time out of 10, it will load up quickly without a problem. 5 times out of 10, it will load up in about an hour. The other 4 times out of 10, my hard drive will not even appear to exist to my computer and it will completely ignore it in Disk Utility, Networks, and there is no icon in Finder or on the Desktop. A few weeks later I will try again, and it will work. Basically, my ibook G4 will not see my hard drive. The hard drive is on, it works on my Windows Desktop just fine, but my mac can't see it. Restarting my computer helps sometimes. Is there anything I can do to find the hard drive without restarting?