can I back up an external hard drive to another external hard drive using time machine?

Potentially, yes. Time Machine (TM) can back up your MacBook Pro's (MBP) internal drive and any drive directly attached to it. It can back these up to either a Time Capsule (TC) or to another local or network drive.

So, to help with some step-by-step directions, how are you performing backups now?

I have photos on an external hard drive and would like to back it up to another external drive

Yes....IF......

1) The photos external drive is connected directly to your Mac via USB, FireWire or Thunderbolt and formatted in Mac OS Extended (Journaled)

2) The "other" external drive is your normal Time Machine target drive for your Mac's backups

If both conditions above are met, click on the Time Machine "clock" icon at the top of the Mac's screen

Click on Open Time Machine Preferences

Click on Options

Click on the photos external drive to highlight it

Click the - (minus) button at the bottom of the list to remove the drive from the list of items that are excluded from backups

Time Machine will back up your Mac normally and now back up the photos external drive

You would be better spending a bit of money on a 3rd party backup software like Carbon Copy Cloner or super duper and you can set it to create a clone of the external disk with incrementals once a day or even less frequently if you are not updating often. Leave Time Machine alone as it is fragile enough without messing with its settings every day.

The advantage of the clone is files are directly accessible without needing to be restored by Time Machine.. ie both drives are identical clones of each other.

1. It depends on what format that drive is in.

2. Open the Time Machine pane of System Preferences, click on Options, and if possible, remove it from the exclusion list.

(158771)