Recently, two incidents:
- 2012-10-23 18:49 only eight of nine were at the Computer level in Finder – the ninth (child file system backups) was found by a mount command but not present in Finder; children of that missing file system were present
- another in which maybe only seven of nine were at the Computer level.
That second incident might have preceded the symptoms noted under root file system not mounted, some children mounted.
Whilst I didn't investigate thoroughly on either occasion, my gut feeling is:
- simple mount point conflicts were not responsible for the problems.