How do I backup an external HDD using TimeMachine? I have used Time Machine for years to backup my main HDD, but I would like to back up an external drive at the same time. Normally you would remove an external drive from the Exclude list in Time Machine Options to have it backed up, I have done so but Time Machine still ignores this drive.

How do I get Time Machine to include this external drive in its backup?
Make sure the external drive is some flavor of Mac OS X Extended format. 

If it's not, wipe it and reformat it or use another tool to back up those files.

Once that's done, make a couple of sample files on the drive.

touch /Volumes/whatever/test1
touch /Volumes/whatever/test2
tmutil isexcluded /Volumes/whatever/*


You can see what exclusion rules might be causing the test files to be backed up or not backed up. The manual page for tmutil will show you how to add and remove exclusions that the UI won't show you. 


On OS X, what files are excluded by rule from a Time Machine backup?