Hi Brendan,
first of all: Don’t mix Harddrives! Alle Harddrives must be exactly the same Brand, size and firmware to work properly in a hardware array.
Your problem might be a hardware one. The slot #3 could be gone. I’ve had a simolar problem with a sonnet 4-bay raid system. I’ve sent it back and got a replacement from Sonnet. It took about 2 weeks.
To test it, you have to replace the drive of slot #3 with exactly the same HD you used before and then rebuilt. Don’t mix Hitachi drives with Western digital!
cheers
Rainer
factstory
Rainer Wirth
phone_0049-177-2156086
Mac pro 8core
Adobe,FCP,Avid
several raid systems