You are correct, any area in SmarterMail that allows for a path designation can have an associated Volume Mount. You can use the Volume Mounts to mount anything you would normally mount in Windows or Linux including shares, new drives/partitions, iSCSI mounts, etc. I recently used this page to load a new iSCSI mount shared from a NAS, create and format a partition on it, and then mount it as pathing for Secondary Storage which we also recently added to domains. 

But wouldn't that put ALL archives on the specified volume?

I was kind of looking at keeping 3 months locally and say 9 months on an external drive.

It's RARE a client needs emails restored from long ago (but it happens), but having to restore info from the most recent few months is commonplace.

Thats exactly what the Secondary Storage does :)
It works for Email and File Storage atm, recently accessed mail and files will be moved to "active" (current) storage until they fall behind the specified time, and then will be moved to secondary.

Ok, I'll have to look into all that.   The Volume Mounts documentation, is kinda lacking in "application". :)

It's a new feature, and a new way of doing things, so the initial documentation was an explanation of the different settings. Application has been added, it just isn't published yet. In addition, it's difficult to offer a comprehensive list of everything it can be used for. If you've mounted drives, etc. before on a server, then put them into use, this is essentially the same thing, it's just automated through SmarterMail. 

What happens if the mounted volume fails?  How Fault Tolerant is this feature?

If a volume mount fails to mount we note it as having failed on the Volume Mounts page. This is the case for newly added and existing mounts. If one of your mounts were to be taken offline later - I expect SmarterMail will react the same way it does if a hard drive goes missing in that domains or other data on those volumes will fail to load and generate errors. This is just hooking into the existing mounting functionality built into your server's operating system, so should behave no differently. 

So, if a Volume Mount fails and then a day later it is brought back online will the Secondary Storage be brought "up to date" to be in a consistent state?

David,

In that scenario, any domain that uses that mount for secondary storage should be brought back up to date when secondary storage is next processed as part of the nightly tasks.