kernel panic with ? List valid disk boot devices, even tho the disk is listed?

[Jaye Mathisen]

Trying to boot from a CF card.

Kernel loads, but root mount fails.

Nothing I try at the "mountroot>" prompt seems to work.

pccard: card inserted, slot 0
pccard: card removed, slot 0
IPsec: Initialized Security Association Processing.
ad0: 122MB <SanDisk SDCFB-128> [980/8/32] at ata0-master BIOSPIO
Mounting root from ufs:/dev/ad0s1a
Root mount failed: 6

Manual root filesystem specification:
  <fstype>:<device>  Mount <device> using filesystem <fstype>
                       eg. ufs:/dev/da0s1a
  ?                  List valid disk boot devices
  <empty line>       Abort manual input

mountroot> ?
panic: Root mount failed, startup aborted.

syncing disks... 
done
Uptime: 3s


This is with 4.8-stable, supped as of last night.

Thanks for any advice.

A disklabel -r shows the partitions as being there...

Any tips appreciated.