Any way to make paths "optional"

I have a Linux client that I sometimes have an external drive attached to. When this drive is attached I’d like to back it up, so I added the path. The only problem is now when the drive is removed I get a “Cannot access path to backup: Errorcode: 2 - No such file or directory” error. Is there a way to make this path “optional” so that it doesn’t fail the whole backup if its missing?

Section 8.3.4 of the Administration Manual shows how to set the optional flag, or any combination of flags, on a backup directory. You need to give that directory a nickname and then append the flags. For example: /home/tommy|Tommy's stuff/optional marks /home/tommy with the nickname “Tommy’s stuff” and then flags it as optional. No error should result if the tommy directory is absent.

Thanks! For some reason googling didn’t bring that up for me.

Now my next question is how do I add a flag to an existing directory from the command line. Googling that also turns up nothing for me, hopefully it isn’t also super obvious and in the admin manual.