All SCSI drives need unique IDs assigned to them, and single channel cards usually have IDs 0-6 available (the card is 7 by default). Depending on the drive cage backplane and SCSI controller, this can be set either manually by jumpers on the drive or automatically assigned by the backplane. In your scenario it sounds like you will need to set the jumpers yourself.
The good news is that if you need to use manual jumper settings it should not matter what slot the drives are plugged in to. Youcan even change the IDs of the drive so long as you set your existing drives to 0 and 1 ? just make sure that the SCSI card BIOS is looking for SCSI ID 0 as its boot device (this is usually the default). If you are using an Adaptec adapter select ALT-M during boot up to access these settings.
You may need a new tape for these drives by the way. Most systems ship with a dual drive controller tape, some with a tape that has enough slots for all of the potential drives. If you have not checkedmake sure you take a peek before shutting your system down to perform the upgrade or you may get a nasty surprise.
Also make sure that you have documentation for all of your drives on hand. You will most likely pull some hair out getting all of the jumpers and termination settings correct if this is your first run at SCSI settings.
As always, make sure you have a solid system backup and allow plenty of time to get things fine tuned. Good luck.
