Original scsi controller chip. Can be a little quirky.
Checked 5V is going into and out of that diode? It is the most likely cause.
Also worth checking voltages, if you got low 12V or 5V that would affect the drive. Probably OK.
You might want to put a little DeOxit in the scsi controller socket, that might do the trick. If you're going to clean, do a little maintenance on the sockets too. Doesn't hurt putting some on the pins that run into ribbon cables, but do let it dry and wipe before putting cables back on.