I've bought an HP Ultrium external SCSI LVD tape drive.
I already have an adapter in my server, but I need to know if I have to shut down and power off the server before I can connect up the tape drive?
If in doubt, shut-down and turn off the server before connecting or disconnecting.
The HP server spec will say if a particular SCSI interface supports hot-plugging. This is often abbreviated to "HP" or "non-HP" (which can cause confusion).