BLZBNTAGT00000BB8 when updating

Hi -
16GB Apple M1 (Big Sur 11.1-5) here with all 3 games installed Classic, TBC & Retail on an EXTERNAL Thunderbolt SSD.
Everything works fine until an Update shows up, at which point Upgrading, or Repair for all 3 installs fails with a BLZBNTAGT00000BB8 code.
I made the external drive as permissible as it can be permissions-wise, everything looks good but Updates just won’t work. I tried all the suggestions around this issue already - none of them work. My only alternative is to delete the game and re-install it when an Update comes out - That’s obviously not sustainable. Any help here Blizzard ? I’m happy to do a Screen Share and let a Support/Dev expert poke around. Thanks much.