Hmm... looks like /dev/sda does not exist, yet. But somewhere under the /dev tree there are probably the right entries. Perhaps /dev/scsi or /dev/bus or /dev/disk something. You could just try to let the installation process continue a bit further. When it's about to start the partitioning, the sda devices should be there. When 'ls /dev/sda' gives something, you've got it. Risto