Problem Importing Old Email

[ParishM]

trying to import a mailbox from Apple mail on an old
hard drive. i connected the old drive via USB and booted
to it, then copied the .mbox i wanted to a flash drive. i
copied the .mbox to my current drive but when i tell mail
to import it i get an error saying no valid mbox files found.

if i open the .mbox i can see all the individual messages
and attachments so i know it's all there but why won't
mail import it ?