this will be maybe dumb question, but I want to be sure before starting doing anything. I have here Dell PowerEdge 2800 and needs to put into it another disk. The problem is that I took this drive from another server and now needs to do a image of it. Problem is that I wasn't able to unscrew it from the tray and it doesn't fit to the Dell's one. Can I take the SCSI cable which is going from motherboard to the back of the trays and put it directly to the one disk drive?
Without a lot more information I'd have to say that you almost certainly can't do what you're trying without a lot more knowledge of the disk and the system board. The reason is that most if not all SAS/SCSI backplanes have auto-termination and ID hardware built into the backplane/disk-ports that won't be replicated by simply 'shunting' the cable. If this was SATA it would be a different matter and much easier, are you sure it's a SCSI disk?