External Harddrives not mounting

Hey..
I have a MacBook and a little while ago my 320gig wd external harddrive stopped showing up on my desktop.It was working fine for about two months and then nothing.I did some searching and did not find what could be the problem..I checked disk utilities and it was not showing up there either..I tried using fire wire instead of usb and it worked so I figured it had to be with the usb connection in the xhd..Well I got a 500gig Kaser hard drive and it worked great until yesterday when it stopped mouting..not showing up on my desk top or in the dusk utilies.. So I really dont know what to do.. btw I never formated either hard drive for my mac since I use the had drives on pc's too.
Thanks

Does it mount on a Windows machine?

i'm having a problem myself... i have a 120GB External HDD. when i hook it up to my windows machine , it reads the files & if i keep my hand on it, i can feel the HDD spinning inside... when i hook it upto my powerbook, nothing happens & i cant even hear the HDD's spinning hum or feel anything, though the small little light comes on stating that its on.

what could be the problem, i have some large files to transfer.
Any help will be apprciated.

I have a 1.5TB external drive that sometimes decides that it's going to drop out on me. I have to turn it off/on a few times and pull the cable in and out before it will come back and be visible on my Mac (which it is formatted for). No idea why.