Try changing the BIOS settings and resaving them, that may be what the checksum error is about. As for the disk boot failure.. are you sure the disk in your A: drive is a bootable DOS disk?
What model is your bridgeboard? You mention a sidecar - if it's an 8088-based board, you'll probably have a 360K 5.25" drive connected, so you'll need a DOS disk in the right format.
