Got it! Found this post (http://funwithosx.blogspot.com/2009/01/remote-backups-to-network-attached.html) that gave me the key. Being unfamiliar with all the nuances of Mac, I assumed that a Finder connect to the server using smb:// would do the trick, but I needed to connect using afp: protocol.
So I learned I have to open Finder, do Go/Connect to Server, and type in AFP://<servername>, then authenticate to "enliven" the connection to the backup volume.
Then, start the backup, and it works great!
I'm able to do this. I have two Mac Mini computers running Mountain Lion. One is physicaly located in another state and connected via VPN. The hard drive is a NAS solution from Western Digital (My Book Live), and it's on the network.
The remote computer connects using VPN (L2TP), and is able to see the shared directory of the backup drive. That directory is selected as the Time Machine backup for that remote machine.
The VPN seems solid (the remote computer can stay connected for hours at a time), and the backup does work.
Problem is: The backup happens very slowly ... 2-5 days (estimated), to backup about 35 gigabytes of disk space.
It's going to be more effective to just get another drive and have it next to the remote machine instead of trying to always backup over VPN.
I'm impressed that it works though - it's just not very fast.
Wow, 35GB!? That's telling me you are waiting too long before backing up. The bulk of the system should already be backed up before you do it over VPN, then the incremental is small. Although I have noticed a lot of hotels that call it "high speed Internet" are using the term from the 1990's - that is to say if it is 56K or faster they call it high speed. The last hotel took about 2 minutes to load up Google's home page (the plain www.google.com), so forget about backup.
I should add, my application is a mobile laptop - the primary backup is on a cloud drive at home (Western Digital MyBook Live - love it!), so when I travel periodically, I want to keep the backups going. A bit gun shy - last year the hard drive died while I was in Texas, and the Apple store there didn't have any replacements in stock (had to get it from them because it was under warranty).
I'm curious though - if you use 2 different drives to back up to, does that work well? When you switch drives, does it pick up where it left off, or start a new backup set (sparse bundle)?
Sure. 35GB is not a lot of space for me actually. That can easily be a day's worth of work. I agree though, it's best to backup your machines as much as possible to keep the increment small.
I love the MyBook Live too.
Using two drives works great! It's one of the many praises I sing about OSx. Yes - finally something great to talk about on the forum ... LMFAO!
If you use two drives, you have some choices. TIme Machine will detect whether you have both drives connected at once, and give you an option:
1. either migrate your backup from one drive to the other, or
2. use both simultaneously - in which case, Time Machine will alternate drives.
If multiple drives are not connected at the same time (e.g. one at home and the other is at work), Time Machine handles each drive individually.
I've never had a problem.
I think you can have more than two drives as well that you back up to.
Check out the docs. ...