Sigh.

It turns out there’s actually a nominally supported way to address exactly this scenario – tmutil associatedisk (kudos to Simon Heimlicher for documenting this).  From the man page:

   associatedisk [-a] mount_point snapshot_volume
           Bind a snapshot volume directory to the specified local disk, thereby reconfigur-
           ing the backup history. Requires root privileges.


           In Mac OS X, HFS+ volumes have a persistent UUID that is assigned when the file
           system is created. Time Machine uses this identifier to make an association
           between a source volume and a snapshot volume. Erasing the source volume creates
           a new file system on the disk, and the previous UUID is not retained. The new
           UUID causes the source volume -> snapshot volume association to be broken. If one
           were just erasing the volume and starting over, it would likely be of no real
           consequence, and the new UUID would not be a concern; when erasing a volume in
           order to clone another volume to it, recreating the association may be desired.


           A concrete example of when and how you would use associatedisk:


           After having problems with a volume, you decide to erase it and manually restore
           its contents from a Time Machine backup or copy of another nature. (I.e., not via
           Time Machine System Restore or Migration Assistant.) On your next incremental
           backup, the data will be copied anew, as though none of it had been backed up
           before. Technically, it is true that the data has not been backed up, given the
           new UUID. However, this is probably not what you want Time Machine to do. You
           would then use associatedisk to reconfigure the backup so it appears that this
           volume has been backed up previously:


           thermopylae:~ thoth$ sudo tmutil associatedisk [-a] "/Volumes/MyNewStuffDisk"
           "/Volumes/Chronoton/Backups.backupdb/thermopylae/Latest/MyStuff"


           The result of the above command would associate the snapshot volume MyStuff in
           the specified snapshot with the source volume MyNewStuffDisk. The snapshot volume
           would also be renamed to match. The -a option tells associatedisk to find all
           snapshot volumes in the same machine directory that match the identity of
           MyStuff, and then perform the association on all of them.

Perfect – and I particularly like the subtext of the prose, which seems to be a subtle acknowledgment that this is a thing that happens frequently, and that macOS’s default behaviour is stupid… “recreating the association may be desired”.  No shit.

Unfortunately, that command doesn’t work in Mojave.  I’m apparently not the first person to notice.

It appears the tightened security, and in particular expansion of SIP to cover many more parts of the system including Time Machine backups, are to blame.  Even granting tmutil Full Disk Access etc in the system security settings is of no use (contrary to the stated purpose of Full Disk Access).

So you have to disable SIP first – which requires a reboot, obnoxiously – and only then does tmutil work again.  You’ll want to enable SIP again once you’re done, most likely, as the protections it provides are useful – it appears tmutil nve eeds to be updated to account for the new protections.

Luckily I’m far from the first person to need to create RAID and/or encrypted CoreStorage volumes, in El Capitan.  Florian Knapp has a concise summary of how to set up an encrypted RAID volume.  Tom Nelson (of About.com) has a slightly more detailed tutorial for managing the RAID part.

Now I just wish the hard drive industry would actually push capacities up, like they once did, so that I don’t have to resort to striped RAID sets just to make a disk big enough for Time Machine backups.  It feels like we’ve been effectively stuck at 6 TB for many years now, and affordable 8+ TB drives aren’t really on the horizon (Seagate & Western Digital have offerings, but historically have been bad brands for drive reliability, e.g. Backblaze’s data, plus my own personal experience with their drives).

An internal state error occurred
Operation failed…