How can I store 15 years of older MacOS Mail on an external drive? I started with Eudora files and converted them to Mac Mail files. Now I have the Eudora files, which I can easily locate onto an external disk because they're flat files.

But I also have that decade+ of converted mail taking up a huge amount of room on my Mac.

I don't want to delete the Mac Mail files, but I want to move them off of my computer and onto an external disk. They are in mailboxes since the Eudora files were sorted into mailboxes.
There are a number of existing questions/answers that address this very topic:


Archiving mail from Mail.app
Archiving Apple Mail to an External Drive
Move mail boxes data to external drive on Mac Mail on Mavericks
Moving individual Mailbox to external drive


These posts should get you on the right track to migrating your mail over to the external disk.

Basically, what you need to do is either copy/move the mbox file to the external drive or use the Export function in the Mail app to export mail to a mailbox located on the external drive.