I am using three Macs (two iMacs and an MBAir) and I am keeping all my documents on an external USB Hard disk which I plug into whichever machine I am using.

I am running Mail 3.2 under Leopard (10.5.2) on all three machines, using an IMAP account.
While IMAP takes care of synchronizing my emails on the server, I have about 150,000 messages stored in local mailboxes. What I would like to do is to have Mail store these local mailboxes on my external USB drive.

What I did was copy the contents of ~/Library/Mail/Mailboxes to a Mailboxes directory on the USB drive, deleted ~/Library/Mail/Mailboxes, and created a symbolic link that links ~/Library/Mail/Mailboxes to /Volumes/USB/Mailboxes. Then I deleted Envelope Index and started Mail, which reimported my 150,000 local messages, created a new "Envelope Index" file (about 70MB) (and an "Envelope Index-1" file (about 300K) and everything worked great.

My problem is that while this solution works at first, after a few times of moving the USB drive between the three computers it stops working, even though I made no change at all to my local mailboxes. When I display the messages in a local mailbox or I search in Mail, the messages turn up fine, but when I open a message, a different message opens, not matching what was in the Mailbox or Search list. Interestingly, Spotlight still works fine and can locate the correct message (i.e. the correct message opens when I click on the Spotlight results). In order to make Mail work again, I must quit Mail, delete the Envelope Index files, and reimport all my local messages, which takes a long time. I imagine the problem is that after unmounting and remounting the USB drive, the information in Envelope Index somehow does not work any more.

I have the following questions for someone that understands how Mail stores and indexes messages, and how volumes get mounted:
1. Any ideas about how to avoid the problem I am having with the Envelope Index getting "out of sync" with the mailboxes on the USB drive? My USB drive has a standard Mac OS filesystem.
2. Is there a better way to have Mail store messages in an external drive?