Can't start up from an external drive in Catalina.

I have a nightmare on my hands!

I just bought a new 2020 MacBook Pro (13"). It came pre-loaded with Catalina (no surprise). I ran Migration Assistant to move everything over from my old 2015 MacBook Pro (Retina).

After doing that, I used Carbon Copy Cloner to clone my old Mac to an external drive (the old Mac was running Mojave). I then erased the old machine and sold it. Later I discovered that Migration Assistant did not move my MailData file over from the old machine to the new machine. I had just lost about 20 years of email messages that were stored "On My Mac"!

No sweat, methinks. I'll just boot the new machine off the Mojave external drive (via USB). No such luck! Catalina won't let me set the external drive as a startup disk in system preferences (something about an inability to "bless" it. I then tried rebooting with the option key held down. I selected the external drive and I got the error icon (circle with a diagonal bar through it).

I'm sure the email messages and accounts are still in the ~/library/mail/V2 folder on the external drive, but I can't get to that folder.

If I could boot off the external drive, I could simply start Mail and export everything. If I could reach the Library folder on the external drive, I could drag the MailData file to the new Mac. But so far I can't do either.

Any advice would be greatly appreciated!!

Related question: it appears that Catalina stores email in a 'V7' folder rather than a V2 folder. Assuming that I can somehow get to the V2 folder on the external drive, can I deposit its MailData file in the V7 folder on the new Mac without problems?