"i was afraid that that was the case. i bought a portable hard drive to back my computer up with. I definitely learned from my mistake of not backing my computer up. All my music, school notes, etc are lost."
Important questions:
The "enclosure" you bought -- is it a "portable" enclosure that connects and gets its power via USB? With or without a power supply? (This can be a VERY important issue)
Be aware that the USB ports on many Macs (including laptops) have a slightly "lower power specification" than do those on PC's. Not sure why Apple does this, but it -is- true for some Macs.
What this means is that when you connect a USB drive to some Macs (and the drive is being connected WITHOUT an external power source), the Mac's USB port may not supply enough current to get the drive "up and running" and mounted on the desktop. In some cases, you may hear it "half-run" -- that is, spins up a little, then spins down, up, down, etc. This is happening because the drive can't get enough power.
The solutions are to use an external power source with the external drives. Sometimes a "powered" USB hub might help. Another solution is a "USB Y-cable" -- has -2- connectors at the Mac end, merging into a single connector at the drive end. This provides a little extra power and gets the drive going.
Having said that, there is another way (one much easier in my opinion).
That is, if you can spend $20 or so, get one of these:
http://www.amazon.com/s/ref=nb_sb_noss?url=search-alias=aps&field-keywords=usb+sata+dock&x=0&y=0
(many items shown, they all work the same, just pick one you like that's cheap)
Put the bare drive into the dock, connect the dock, turn it on. If the drive still won't mount up, it's probably the drive.
You'll find that a USB/SATA dock is a -very- handy piece of equipment to have in your drawer, and did I mention that they were -cheap-?