Lol I just had a similar problem with my HDD last night. After transferring my entire collection of wii games to my HDD it literally just crashed and i had 160 GB's of unallocation space. I went into Diskpart and formatted it from there and readded the partitions. Just because windows explorer doesnt see it doesnt mean windows doesnt recognize it. Its there but invisible cause you cant add anything to it.

 

I usually connect my USB devices by what I think is a port splitter and they work fine. But for this HDD I tried the normal USB ports, EVERY single one and I even tried connecting it as an internal HDD. Nothing worked.
I tried Computer Management Tools (again) but the HDD doesn't show up and I even tried to update the drivers but it says they're up to date.

Thanks for your replies, any other ideas?

 

Even taken from the enclosure/adapter and connected internally and it still doesn't show up but loads fine in another computer... shesh

2 other longshot ideas:

1) Check and see in device manager if there are any error marks or unknown devices, I think you can also force it to show unattached devices as well, maybe the driver got corrupted, or just removing it and letting it re-identify the device will help? Might be a driver for the HD itself and the ata-USB bridge

2) Try an Ubuntu live CD/USB and seeing if that can recognize the drive, sometimes linux can see things when windows is being a b!tch.

Dumb question (since I don't know about the wii and external drives to load backups): was the drive by chance formatted in a special way to load the backups after you initially connected it to the comp and this particular comp hasn't had the proper software to read the drive whereas the other comps you have tried it with have had the software installed?

Good luck.