So today I got to do some work on recommissioning an old HP Proliant ML350 G4 that I bought for peanuts on an auction site. I already had it booting up, but the SCSI backplane was not correctly cabled.
First, I connected the power cable to the SCSI backplane. It started to go through the motions of detecting drives, etc. However, the drives were not showing up at boot time, and when I tried to boot from a Live CD (CentOS 5.5 LiveCD or HP SmartStart CD x64), it would not see any disks and, what's more, it would hang with the hard drive activity light blinking rhythmically.
After some fiddling, reading of manuals, and contemplation, an idea came to me - to try reversing the SCSI data cable between the motherboard and the backplane. I unplugged it, turned it around, and plugged it back in. After that, it started to work. I was able to boot the machine from both the CDs mentioned above.