This was a Big Deal for me.
This may help someone else.
I used root to compress the /var/imap AND /var/spool/imap folders, then moved to the new drive in the same locations the clean install created.
Permissions were lost, -my dumb-, (and you can’t use Server Admin to repair permissions for _cyrus or /var as I found.)
Cyrus crashed with a permissions error. Disk Utility Repair Permissions laughed heartily.
Sigh.
Heavy sigh.
Net searching turned up:
http://support.apple.com/kb/HT3120/viewlocale=en_US
Which helped.
Halfway down the instructions for completely rebuilding imap 10.5:
sudo chown -R _cyrus:mail /var/imap
Then I added:
sudo chown -R _cyrus:mail /var/spool/imap
Then ran:
/usr/bin/cyrus/bin/reconstruct -i
And the multi-gig database and users were alive and kicking.
Thanks for listening.
😉