My newly installed dell PowerVault LTO4-120 Drive (in a PowerEdge T300/Server 2008R2) displays a code "6" and the exclamation indicator is lit up.
What do these mean, and how can I fix them?
You've got a PowerVault LTO4-120:
In short, try a different cartridge. If that doesn't work call Dell support. There's a good chance you have a bad tape drive (assuming you've tried multiple tapes and they're all returning that code).