Dude:

This souds like improper termination or improper SCSI ID setup.

The last SCSI device on a daisy chain has to be 'terminated' (a jumper must be installed on the drive which informs the system that the device is last in the chain). If the jumper is set correctly for one drive only, and you add another you will have to move the jumper on the first drive because it's no longer the last in the chain, the second drive is now last in the chain.

Each SCSI device must have a different SCSI ID. On removable drives this is set with a small dial, on fixed drives it's achieved with a jumper.

The manufacturer's web site should provide that info.

Reverend