I just got it back to working!!!

The problem was that my scsi-controller comes with a built-in power supply for the hard disk. So I connected the hard disk power connector to that power supply.
Somehow, there did not seem to be power on that supply. So logically, without power the hard disk won't work. I now connected it to the main power supply and it is being recognized. I think I'll manage it from here.
Thanks everyone for your help so far.