I can not find my external harddisk

Hello,

I've got a new, unused external harddisk. It has it's own powerfeed, and goes into USB.

A friend got it for me (he knows his computers!), it's a harddisk he had for which he bought a cover.

It says it's pc and mac compatible, and my friend tested it, and it worked on his windows pc. He said all I needed to do was to confert it so it would be mac-compatible.

Today I wanted to do that, but I can not find de disc anywhere on my macbook! It does not show up in Disc utility or anywhere else I checked.

So I figured it was maybe automatically conferted to a windows format when my friend plugged it in to test it. That's why I tried my husbands windows-laptop, so I could maybe configure it there to something my macbook can read. But the disc doesn't show up anywhere on my husbands laptop as well!

The harddisc is turned on, the little green light says it's on...

What can I do to find it?

p.s. I'm still running OSX 10.6 (that's why I need it. I want to backup my documents before updating to the newest OSX)

Thanks!

Jelka