I don't like the idea of booting Windows from an external HDD. If it does boot, confusion might arise when doing a reboot owing to the presence of the internal installation. It might grab the wrong BCD, I think. But maybe the risk is less in a Legacy installation. I don't know.
(1) But my first thought is -- did you set BIOS to boot in Legacy Mode &/or enable CMS? BIOS must match the mode of the installation.
(2) The fact that Windows (not IBM) Boot Manager was named in the error message is interesting. It sounds like the VBR (volume boot record) of that Windows partition still wants to invoke the Windows Boot Manager.
(3) Obviously, there is a massive hardware change. But I thought Windows was supposed to switch the drivers automatically. Maybe it refuses because the drive is external.
(4) Trying to repair it in a recovery environment fails, I think, because it wants to repair the internal installation, not the external one that is broke.