Tobias,
You don't mention what OS you're running this all on. What probably happened is the drives reconencted in a different order on the SCSI bus.
The best way to prevent this (in Windoows with fiber cards, anyway) is to enable persistent bindings on your host adapters and lock the drives in to specific SCSI addresses.
Not sure how to do this with non-Windows or iSCSI setups. It is somewhat HBA-specific.
Ken