Assertion failed: mdrcp->colnamep->start_blk <= rcp->un_orig_devstart, file ../common/meta_raid.c, line 151
Abort (core dumped)
No documentation about this error available anywhere, and a Google only found 3 or 4 hits, none of them helpful (one of them involved using LD_PRELOAD to replace the abort() function to allow 'metaclear' to delete the RAID, recreate it, and reload from backups.
I worked out why the error occurred, though. When the disk was replaced, it of course came with a label/TOC with partitions defined. If these partitions don't match the pre-existing RAID setup, the metadisk tools die a death.
All that was required was to build up the proper partitioning, and then everything worked fine.