The BeOS does not at this time support any of the various SCSI adapter cards, just the built-in (on the logic board) SCSI interfaces that all Power Macs have.
So in order to get the BeOS to see any hard drive in a Power Center Pro, it has to be connected to the internal or external built-in SCSI interface.
Since the Power Center Pros ship with their internal hard drive connected to an Adaptec SCSI card, to get the BeOS to see it, you need to open the case, and unplug the drive from the SCSI cable that goes to the Adaptec card, and plug it in to the cable that goes to the CD-ROM drive (which is the internal SCSI connection cable). It's easy to do, as there are extra connectors on that ribbon cable for just this sort of thing.
You do give up the extra performance of the fast/wide/ultra/whatever interface on the Adaptec (or other SCSI) card, but with the speed of the BeOS file system, you will still think it's fast! ;-)