external hard drive not being recognized

Have you shut down (powered off) your Mac since this problem occurred (with the drive powered off at the same time)? Did you try a different USB port? If it is connected to a USB hub, did you try it connected directly to the Mac's port with no other USB peripheral devices connected (except for your standard keyboard/mouse)? Did you try a different USB cable?

When you run Disk Utility, does the drive appear in the sidebar?

A bad USB cable is a common problem. I would try getting a new one to see if it fixes the problem, before investigating other possibilities. Make sure it is a USB 2.0 cable, and not any old USB cable you may find you your storage (the older ones made for USB 1.1 are often not rated for the higher speed).

I have a similar problem. Just bought a Buffalo 1 TB 1SATA, External USB 2.0 Hard Drive. It has its own power supply. It is all connected correctly, power on, heard the sound when it started. I can see it in Disk Utility and System Profiler, but it won't mount. I think it may be a problem with the connection ID, as it says Unknown. I've tried Verifying the disk, says its OK. Tried Repairing disk, and it keeps on thinking (forever) until I stop it. How can I get the **** thing to MOUNT! Any help please.

Does the printer work off the same USB port you are using with the hard drive?

Actually, if you connect your keyboard/mouse to the same port, do they work?

I'm having the same problem but I have a Seagate FreeAgent. Shortly after downloaded the newest Mac update my drive started disappearing. Usually a reboot would solve the issue.

Do you have another device that you could hook your external drive up to? I wanted to see if it was the drive so I hooked mine up to my Xbox 360. The Xbox had no problem seeing and accessing the drive. Makes me think that once again, an update to OS X has killed the functionality of my iMac.

I know this does nothing to answer your question but it does hopefully show that you are most certainly not alone.

I'm sorry I didn't see your post showing you hooked it up to your Windows XP system.

Clearly we do share a lot of the same issues in common.

Sorry for posting yet another response to this thread. I recalled reading on TUAW that they asked if anyone was having issues since updating to 10.5.7. As I made my way through the 10+ pages on comments I noticed a very common theme.

USB mounted external hard drives disconnecting, not being able to eject, or not able to read.

Dozens of comments having the same issue. Looks like it almost has to be the last update. A couple of responses suggest installing the Combo Updated for 10.5.7 to see if that helps. Others mention it might have been an issue with the newest iTunes update.

i'm getting the same problem.
worked perfectly last nite. now it's not mounting.
disk utility says it appears to be ok.
tried my freeagent go and it mounts fine.
then when i try to eject it it says it can't be unmounted.
what the **** is up with these updates.
all they do is make more problems than fix them.
it's like duur we got a new build let's rls and not check it against other conflicts.