external HD formatting issues

I plugged a brand new out of the box WD 640Gig drive into an Antec enclosure and my PC didn't detect it.

I looked in device manager and disk management.

I'm guessing that it's because i didn't reformat it first.

I would plug it into my MoBo but, i already have two drives (one system one just media) plugged in and no more sata connectors or power supply cables to power a third drive.

What is the best way to tell what drive is which (they are identical) I don't want to accidentally unplug my system drive.

One is plugged into the Sata 0 port the other the Sata 1 port according to my MoBo. Is there anyway to tell within XP which drive is which.

If I unplug the wrong drive will stuff get messed up?

or should i just skip the hassle and get a local shop to format it for me.

If I understand, you would like to disconnect the existing "just media" internal HDD temporarily and hook up the new one instead, just so you can Partion and Format the new one before returning it to the external enclosure. Trouble is, trying to identify which internal to disconnect.

Not really a problem. Turn off and disconnect the power cord. Open the case and disconnect BOTH the data cable and the power cable to one internal drive. Close the case, reconnect, and turn on. Two things could happen:
1. It boots and runs just fine. So now you know you have disconnected the "just media" drive and can proceed with your plan. When done, you just re-connect it.
2. It fails to boot and asks you to insert a bootable disk somewhere. Now you know you chose the wrong drive. Shut down, disconnect power, open case, reconnect first drive, disconnect the other, close up, power cord in, and reboot to check. Should end up in Situation #1 above.

Just disconnecting a drive like this should do no damage at all to your system.