This thread had started with a root-on-raid problem, where the disklabel was not being acquired properly.
Ken Westerback determined that I'd had a disklabel marked as Version 1, but I had values from Version 0 for my failing partitions. Editing the disklabel and replacing fsize/bsize/cpg seemed to resolve my issue. I now think I understand exactly how the old partition data structure ended up in a new format partition; the affected partitions were restored via newfs/restore, and that is the most likely culprit.... as the restore was of a pre-ffs2 environment, executed from a post-ffs2 system. Thank you, Ken!
