I have two tape-changer robots (Quantum Superloader 3), one with parallel SCSI connectivity, the other with SAS. They were connected to a Centos 5 server, and boot would proceed normally.
They are now connected to a new server running CentOS 6.3. When the server gets to the 'Starting udev' stage, one after the other, the changers begin going through many routines, presumably some sort of inventory. The SAS changer goes first, probably because it gets assigned a lower device number ( /dev/changer-sg4 compared to the other changer's /dev/changer-sg6). While this is happening, the boot process just stays on 'Starting udev'
The first changer churns through its inventory in about 3-4 minutes, then the second changer starts. A few minutes in, I see SCSI error messages appear on the screen (I can paste them here soon if they are needed), but the changer continues doing its inventory and boot proceeds correctly from there. The changer finishes before boot does, and when boot comes up, both tape changers seem to be fully available and working.
The upshot is, bootup completes successfully without manual intervention, but I'd like to cut out the extra time this is adding to boot, and extra wear on the tape changers (if there is any).