external-diskfirewirehardwaremonitoringusb
Disk Utility reports the S.M.A.R.T. Status as not verified for external disks, and by default Mac OS X only seems able to access S.M.A.R.T. information on internal hard drives.
Is there a way to get SMART information on external USB or Firewire hard disk?
Yes, I just got a new Mid-2012 MacBook Pro 13" and did this very thing over USB 3.0. I know of no reason why it would not work on a MacBook Air.
When booting from an external spinning hard drive over USB 3.0 on my new MacBook Pro, the external hard drive sends data back and forth to the Mac about as fast as it would if it were installed internally. I have not specifically measured drive speeds in this scenario; I'm just saying that it performs quite well.
As far as I know, all Macs with USB have been capable of booting from an external drive connected via USB--all the way back to the first iMac circa 1998, which only made use of USB 1.0. Booting from an external USB 2.0 drive, whether a flash/thumb drive or a spinning hard drive, is, however, quite slow.
When you boot your Mac, immediately after the startup chime, hold down the Option key. Your Mac will show you a row of icons of each volume or partition currently connected to the Mac (internally or externally) that holds a Mac OS X installation that you can successfully boot from. Use the arrow keys to select which one you want to boot from, and hit the Return key to boot from that volume.
Yes - you should be able to backup your external drive with the photos by removing it from the "Exclude" tab in the Time Machine pane in System Preferences (in this case the external drive is FCPX):
As Nancy Casas also mentioned, a disk in gray will have to be reformatted for Mac, while one in black will not.
Best Answer
Yes (maybe).
There's an open-source kernel driver OS-X-SAT-SMART-Driver for Mac OS X that will work for some USB and Firewire enclosures. It uses SAT (SCSI ATA Translation) to pass the SMART commands through to the hard drive, so only works if your enclosure supports SAT.
SAT isn't something enclosures seem to advertise support for, but the one I use (WD MyBook Studio Edition) happens to support it, so it works for me. It'll probably be quicker to test the software with your drive/enclosure, than to find out if SAT is supported in the specs.