Did you check that it is suitable for use with a Mac? Such a large drive might have been formated as NTFS, which OS X cannot, by default, write to. Open up Disk Utilities and have a look at the drive (if it appears there) to see how it is formatted. If it is NTFS you can reformat it to the Native OS X file system. If it doesn't show at all then the drive is faulty. I take it you are using a separate power supply for the disk?
G-Tech, LaCie, CalDigit, and some Promise drives come pre-formatted for OSX, but I'd still reformat them using my own system anyway. This is also a great time to make decisions regarding partitioning, encryption, and what other systems you might want to share the drive with.