Make Time Machine-like backups of Photos onto external drive
You will need an external drive formatted Mac OS Extended (Journaled) to use as a Time Machine backup. TM cannot use a NTFS formatted disk as the backup destination.

Unfortunately, Time Machine cannot use an NTFS drive as a backup source, either. Time Machine relies on the Mac OS Extended (Journaled) file system on all drives acted on to do its work. (This isn't strictly true as you can create and backup to sparsebundle files on a network drive mounted with NTFS, but this is not in the scope of your question.)

Thus you need two Mac OS Extended (Journaled) external drives - one for the TM backup; one for the Photos Library. If you know how to use Time Machine, you can stop reading. If not, keep going.

In System Preferences > Time Machine, Click the Select Disk... button and choose this external disk for Time Machine backups. One the disk is selected, click the Options... button at the lower right, which brings up the window where you can exclude items from backups. Normally, all connected disks (internal &amp; external) are backed up, except for the Time Machine disk. You can drag (or click the + sign) and add Macintosh HD (or whatever else you may have named the boot HD) to the list of excluded items. This will enable you to backup only the external HD with the Photos library.