booting off gmirror volume

[bas]

I have the root partition gmirrored on four slices and gmirror reports
that they are healthy and happy. But if I try to boot off two of them
(ad2s1 or ad4s1), the boot loader reports that they are not ufs and
gives up. I can boot off the other two (ad6s1 and ad8s1) with no
problem. I change the boot device in the bios setup. I don't know
where to look for the problem. If the gmirror is complete, I should
be able to boot off any of them, no?

252> gmirror list
Geom name: gm0s1
State: COMPLETE
Components: 4
Balance: round-robin
Slice: 4096
Flags: NONE
GenID: 0
SyncID: 1
ID: 2412948172
Providers:
1. Name: mirror/gm0s1
Mediasize: 263176192 (251M)
Sectorsize: 512
Mode: r1w1e2
Consumers:
1. Name: ad2s1
Mediasize: 263176704 (251M)
Sectorsize: 512
Mode: r1w1e1
State: ACTIVE
Priority: 0
Flags: NONE
GenID: 0
SyncID: 1
ID: 2503325657
2. Name: ad4s1
Mediasize: 263176704 (251M)
Sectorsize: 512
Mode: r1w1e1
State: ACTIVE
Priority: 0
Flags: NONE
GenID: 0
SyncID: 1
ID: 753175732
3. Name: ad6s1
Mediasize: 263176704 (251M)
Sectorsize: 512
Mode: r1w1e1
State: ACTIVE
Priority: 0
Flags: NONE
GenID: 0
SyncID: 1
ID: 4165696522
4. Name: ad8s1
Mediasize: 263176704 (251M)
Sectorsize: 512
Mode: r1w1e1
State: ACTIVE
Priority: 0
Flags: NONE
GenID: 0
SyncID: 1
ID: 960889708