error: Could not unmount disk

So then there's no point in adding the external HD to the Privacy tab in Spotlight (if it would even allow me to do that). Does it sound like it might be more of an enclosure issue?

Let me tell you what I tried this morning. I disconnected the drive and reconnected it using the FireWire 400 port on the enclosure (which I hadn't tried before) and it mounted right away and I was able to easily and quickly erase and repartition it using Disk Utility, without getting the error message: Could not unmount drive. I was able to copy 3GB of files via drag'n'drop to one of the partitions and the files went right through. Everything seemed fine, so I tried using the cloning application again to clone my primary drive onto one of the partitions and the app. stalled as before. Had to detach the external from the computer and then do a manual power down. Had to boot up twice for the external to mount on the desktop and then I was able to erase and repartition it as before, without a hitch. What eventually happens though, is that my computer becomes unresponsive and I have to detach the external in order to restore operation. Might removing the drive from the enclosure and putting it back in again do anything or do you think I should just take a chance on replacing the enclosure?

Bought it three years ago, but except for initially partitioning it when I first got it, it's sat idle, unused and not powered on until about a month when I decided to clone my primary onto it. It's a Western Digital 750GB Caviar Black. I actually got two. The other one I installed internally three years ago and it's been perfect.

The case is an OWC Mercury Elite Pro Classic with Quad interface. It's been plugged in but I haven't really used it until a month ago. You're thinking it might be the power supply?

BDAqua, thanks for all your suggestions and the links. I actually have one of the Jentec power supplies with which there do not seem to be any reported problems. I even tested the voltage from the pins at the end of the cord (where you attach it to the drive) with a volt meter (according to instructions I got from OWC) and the readings were perfectly steady and where they should be. With my ear on the brick, all I hear, when I power on the external, is a faint high pitched whine that gets a little higher and lasts for maybe 2" and then the brick is silent.

No, I've been a little hesitant to do that but I think I'll give it a shot tomorrow. Will let you know what happens.

Well, I decided just to order a new updated enclosure for the drive and see if that works. OWC assured me that I had a 30 day refund policy if that didn't do the trick and they, also, were inclined to think the problem was not with the drive. So if you're still interested in how things turn out, I can let you know when I test the new enclosure. But, nevertheless, thanks for your help.

Here's an update. Received the new enclosure. Before I transferred the drive into it, I unplugged the internal cables from the old enclosure that were connected to the drive and then reattached them. Also, I noticed that one of the cable plugs was not perfectly seated on the circuit board, so I pushed that in a bit. Heard a tiny click.

Using the old enclosure, the drive mounted quickly and I was able to Zero Data on it and erase it using Disk Utility. Was not able to Zero Data on it before. Would hang every time.

Thought I would try a different cloning app than the one I had been trying to use and this time the transfer did not stall. The estimated time, however, was about 6 1/2 hours which I thought was way too long to transfer 190GB via FireWire 800. The transfer rate started at 4.5MB/sec and went up to 9.5MB/sec. So I thought I would try the new enclosure and see if that was faster. I stopped the operation, erased the partition and transferred the drive to the new enclosure.

Tried the same cloning app that worked in the old enclosure and this time it stalled shortly after starting to copy files. Erased the drive and tried the original cloning app that had stalled using the old enclosure. It stalled using the new enclosure also. Erased the drive and tried the Restore function in Disk Utility and that went through. Took 4 hours. Attempted to boot up from the backup that Disk Utility created but I got the circle with the diagonal through it and it just hung there, with my fans running at top speed. Finally had to power down manually.

I'm thinking maybe it's the drive that's malfunctioning. It is still under warranty. Even though I'm not able to boot from the backup partition Disk Utility created, I'm wondering why it was able to transfer 190GB of data when the other two, well respected cloning applications stalled.

CCC and SuperDuper.