Keep all the domains and mail on a separate volume. This make future OS upgrades easy as you can just build a VM with the new OS, attach the mail volume disk to it and then install SmarterMail. No need to copy all that mail data between systems. Having multiple volumes on a VM should also improve your backup and restore times, since your backup software should be able to read/write to multiple VM disks at the same time. Just for the improved RTO, we actually split our mail data across 2 volumes, one for production mail and another just for all the mail archives.

In your case it sounds like you're going from a physical server to a VM and the current server can't handle the upgrade. So then you'll need to robocopy the mail data to the new server. Then if I were in your position I'd do a test migration. Take a snapshot of the VM, install v14 on that server and check for issues before doing the SM upgrade. Then check for issues again before doing the process for real.