Back up external drive with Time Machine

Drive needs to be formatted in Mac OS Extended (Journaled) if you want Time Machine to back it up. That's why the drive is grayed out.

Earlier, I had understood that you already had Time Machine backups on the drive....and Time Machine won't back up that type of drive either.

If you want Time Machine to back up the external, you will need to move all of the data on the drive to another drive, then format the external and move the data back onto the external. Check with Lightroom support before you do this though.....to make sure that the image library is compatible with the Mac drive format.

Does the external drive have previous Time Machine backups on the disk? If it does, Time Machine is not going to back up a drive that already has Time Machine backups on the disk.

In the example below, you can see that the drive named MBP TM, which contains Time Machine backups from my MacBook Pro is grayed out and not available to be backed up by Time Machine again when the drive is connected to my MacBook Pro.

Another way to say the same thing.......you don't want to back up a backup. You want to back up the "original" data.

For example, if you were making a copy of a document on a copy machine, you would not want to copy a copy, you would want to copy the "original" document.

Make sense?

And how is the hard drive formatted? If you are not sure, right click on the drive icon on the Mac's desktop and click Get Info.

Look for Format. What is it?

Look for Permissions. What are they?

Hopefully, the drive is not encrypted or password protected, since Time Machine will likely not be able to back it up.

I have a feeling that when you first connected the hard drive to your Mac, that Time Machine asked if you wanted to use the drive for backups, but you declined.

If you had accepted, Time Machine would have reformatted the drive from its current format, which was likely a Windows format that a Mac can read, but Time Machine can't.

We'll know more when you check the drive Format and Permissions.

All things considered, Time Machine may not be the way to go to back up an image library. Reason.....you cannot "browse" individual images in the Time Machine backup......you have to restore the entire library to get a look at its contents.

But, it's your call.