I have been posting on forums and looking for answers for almost 2 years now...here is the timeline of events and yet NO solution 
2010
I have been using my WD external hard drive on my G5 Mac (OS X 10.4.11) at work for the last year or so with no problems. One day I plugged it in and it suddenly did not appear on my desktop. I tried the hard drive on 3 other macs in the building and also 2 PCs and my PC at home. They all access the hard drive and files just fine. I checked forums for any type of answer and nothing matches my exact problem and none of the suggestions for similar problems have helped. In Disk Utility the drive shows up. The Mount/Unmount button says "Unmount" which I would assume the mac thinks it's mounted. If I click to Unmount and then click Mount it says "The disk could not be mounted. Try running First Aid on the disk and then retry mounting." I then go to First Aid and repair the disk and it tells me that the disk does not need repair.
It was suggested that my mac may not be supplying enough power to the USB ports. So I unplugged everything but my mouse, keyboard, and hard drive out of the 5 available USB ports on my machine. I tried the hard drive on all the other ports. The mac detects the hard drive in both Disk Utility and System Profiler every time I plug it in but still does not mount to access the files. I have tried flash drives on all of these ports and those have no problems and mount fine.
The next suggestion was to run the Apple Hardware Test to see if there were problems with the USB ports. I ran the test and nothing is wrong. Then I was told to try the fix for "Invisible Drives" and that did not work.
After reading suggestions on other forums, I tried resetting the PRAM, which didn't work. It was also mentioned that getting a powered USB hub worked for almost everyone with this problem. But since this is my work computer, I wasn't able to talk my boss into getting one of those yet.
After some time I finally purchased a new external hard drive...another WD....with great confindence that it would solve my problem. I plugged it in and I had all the same problems.
2012
My boss finally got me a powered USB hub. With excitement that I would finally be able to use my hard drive on my own computer, I plugged it all in. Nothing. Same problem as before. The hard drive shows up in Disk Utility and System Profiler and still works on every other computer in existence, but will not mount on mine. Aside from buying a new computer myself (because my boss sure won't do it) I dont know what will fix this problem. PLEASE HELP!
2010
I have been using my WD external hard drive on my G5 Mac (OS X 10.4.11) at work for the last year or so with no problems. One day I plugged it in and it suddenly did not appear on my desktop. I tried the hard drive on 3 other macs in the building and also 2 PCs and my PC at home. They all access the hard drive and files just fine. I checked forums for any type of answer and nothing matches my exact problem and none of the suggestions for similar problems have helped. In Disk Utility the drive shows up. The Mount/Unmount button says "Unmount" which I would assume the mac thinks it's mounted. If I click to Unmount and then click Mount it says "The disk could not be mounted. Try running First Aid on the disk and then retry mounting." I then go to First Aid and repair the disk and it tells me that the disk does not need repair.
It was suggested that my mac may not be supplying enough power to the USB ports. So I unplugged everything but my mouse, keyboard, and hard drive out of the 5 available USB ports on my machine. I tried the hard drive on all the other ports. The mac detects the hard drive in both Disk Utility and System Profiler every time I plug it in but still does not mount to access the files. I have tried flash drives on all of these ports and those have no problems and mount fine.
The next suggestion was to run the Apple Hardware Test to see if there were problems with the USB ports. I ran the test and nothing is wrong. Then I was told to try the fix for "Invisible Drives" and that did not work.
After reading suggestions on other forums, I tried resetting the PRAM, which didn't work. It was also mentioned that getting a powered USB hub worked for almost everyone with this problem. But since this is my work computer, I wasn't able to talk my boss into getting one of those yet.
After some time I finally purchased a new external hard drive...another WD....with great confindence that it would solve my problem. I plugged it in and I had all the same problems.
2012
My boss finally got me a powered USB hub. With excitement that I would finally be able to use my hard drive on my own computer, I plugged it all in. Nothing. Same problem as before. The hard drive shows up in Disk Utility and System Profiler and still works on every other computer in existence, but will not mount on mine. Aside from buying a new computer myself (because my boss sure won't do it) I dont know what will fix this problem. PLEASE HELP!
