Thanks! I will take a closer look at the logic board.

When booting the internal hard disk makes a high-pitched whining sound for a few seconds and then stops. The LED flashes a few times. I thought this was due to the fact the the disk was being powered, but when I disconnect the data cable (leaving the power one connected) it makes no sound whatsoever, and the LED is constantly on. Perhaps this would suggest a hard disk failure, but can a crashed/dead internal disk disrupt a whole SCSI chain? As I mentioned earlier, the external hard disk is still not recognized even when the internal one is disconnected.