to get around a "repair filesystem" prompt at XenServer boot, I had to remount / as writable and edit /etc/fstab to change a mount parameter.
(Repair filesystem) 1 mount -n -o remount /
(Repair filesystem) 2 vi /etc/fstab
found the last entry I made for the USB drive and changed the fsck field (#6) from '2' to '0' to skip fsck on boot.
wrote, rebooted, and XenServer skipped the failing mount and booted.
I used an external USB drive as an ISO repository on a new XenServer 5.6 FP1 installation for a test environment. I wanted the mount to persist, so added it to /etc/fstab with an fsck pass number of 2, so it would be checked at startup. I didn't realize it, but apparently USB devices aren't discovered at boot, so fsck failed and init wanted me to do something to fix it.
I'm sure the correct solution is to add a script to mount the volume at a later init stage, but for now, at least XS boots and I can manually mount the volume and repair the ISO repository.