Your best bet would be to find the FRU and find the IBM documentation. Most of this stuff is described online, if you know where to look.
Off the top of my head, the "serial-esque" cable could be anything from a monitoring link, to a connection to control a hardware RAID controller in the array, to a breakout of some form or another to a panel or device that would've set the options for the device, but these are all wild guesses.
As to the trouble experienced, if you're quite convinced it's all straight-through and nothing's goofily differential ('-III' to me would indicate UW SE, or U2W and so on LVD that should fall back to SE, but I can't remember if there's anything weird to be aware of as to differing pinouts on external connectors/cables)... I'd expect the drives in such a rack to come set for S.C.A.M. by default (SCSI Configuration Auto-Magically), and perhaps that's not working out for you.
(Note that SCAM theoretically disappeared from 'SCSI-3'/'SPI-3,' but since 'SCSI-3' doesn't mean much of anything... that factoid doesn't mean anything. If the drives are jumpered for SCAM, obviously they attempt to support SCAM.)