Well clearly the bootloader is detecting 3 disks, perhaps installing the new one rearranged the ordering.
The error is an indication that the partition, if it even exists, does not have an FFS file system.
At this point I can't diagnose the situation... please consider posting the following information in [code][/code] blocks.
$ dmesg
# fdisk wd0; disklabel wd0
# fdisk wd1; disklabel wd1
# fdisk wd2; disklabel wd2
This of coarse assumes your drives will attach as wd(4)...
