After several days (heartbeat and DRBD are new to me) I've gotten Zimbra working with heartbeat, mostly.

If Zimbra is working off Server-B and Server-B goes down, Zimbra transfers over to Server-A. The problem is that the servers reboot so quickly during a test (less than a minute) that Zimbra is about 90% started on Server-A when it receives a heartbeat command to transfer back to Server-B. Server-A takes a while to unmount /opt and both server's DRBD ends up going to Secondary/Secondary, the shared IP is never assigned again. I end up rebooting both servers and everything comes back up.

auto_failback off is set to off on both servers, and heartbeat is set to prefer Server-A to start with.

I've been pulling my hair out on this one, and these are new servers.
2.66G 64bit Pentium Ds
1G of RAM
1 mailbox (I was still testing heartbeat and haven't setup the mailboxes yet)

Does anyone know what I need to tweak?