[G4] Switching primary HD, need moving messages advice

[keith_w]

I have a MDD G4, OS 10.4.8.

I use a second identical (internal) HD to back up my primary HD onto, 
when backing up.
Worked just fine.

I know that because now my bootable "Extra HD" has taken over and is my 
primary HD ~ as that one got wiggy on me, and was no longer reliable.

Just a week prior to the decision I had backed up my HDs and 'Extra HD' 
was very close to being identical to 'HD1'. But... I am missing some 
list messages I wanted to save before I throw the original HD in the trash.

I use SeaMonkey 1.0.6 for my browser and mail application.
I have several folders in my "Home Account" pane, to direct incoming 
messages to.

The message filters work just fine, but now that I'm a bit behind, 
timewise, and want to retrieve those older messages, what is it I must 
move over from the HD1 to HD2?

I do understand to do this while not online, but what folder, which of 
the sub-folders or other 'entities' am I to move in order to have my 
history messages?

I'm not about to move on this until I get some rational advice, so have 
  at it, folks!

Thanks ahead of time!

keith whaley