The tech dude is absolutely wrong. If you have the drive mounted as a local drive, which a Firewire, USB or Thunderbolt drive would be, you can back it up via TM.
As you can see in the attached image, I have chose to exclude cetain drvies from TM. Going down the list is like this:
Bootcamp is a partition of the internal drive and TM doesnt back up Fat or NTFS drives
Media FW800 is a 2TB Firewire 800 drive
SG Back up is a 1TB partition of a 2TB drive
SG - Pics is the other 1TB partition to the above 2TB drive
Video 1TB FW8 is another 1TB FW800 drive.
By default they are chose to not back up, but I can remove them from this list and they will back up. I am really only interested in backing up my internal drive and my laptop via TM.
As far as tutorial for TM, its pretty simple
When you turn it on for the first time it will ask you to identify the TM drive to backup. It basically wants to utilize the entire drive. So if you plug in a 3TB drive it will want to use all of it. So choose a drive accordingly. I prefer the chosen drive to a separate drive or a partition of a drive with low usage.
So once you choose the drive it does an initial back up. let it do it with default settings of just your internal drive. Then once that is done, go back in and remove any drives on your "exclusion" list so that they also back-up to the TM. Remember, anything on the list WILL NOT BACK UP VIA TIME MACHINE.
Also remember that TM will not back itself up as it would put itself in an endless loop.
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