External storage device

If you do what you are suggesting that will make your photos candidates for deletion from the Time Machine backup the next time it does the garbage collection routine to free up disk space for to be able backup.

A better solution is to get another external disk drive to store your photos on. You can then protect the photos by having Time Machine back them up.

Hi,

I'm back up Photos Library on external HDD separately and delete Photos Library from MacBook Pro.

Then Selects Photos Library at external HDD one(when open Photos first time, press option key to select Photos Library).

Whenever open Photos, I connect external Photos Library back backup external drive.