I just joined the forum because I had the same problem as you. My laptop hard drive had died (I thought) a while back. I bought a USB 2.0 external enclosure (X Media), but could never see a drive letter on my XP laptop, and I would get a "drive can't mount" type error on my Ubuntu desktop.
The USB cable I'm using has two USB connections on one end, one USB connection on the other. Anyway, to make a long story short, I tried what the other poster said - another USB port on my laptop - and it works!!!!
I always use the same port, but trying the other one (and maybe the other USB connection on the cable, I'm not sure) actually worked!
I hope that this simple solution may work for you too.