time machine would back up everything on your drives so if you had multiple locations of photos on a single or multiple drives in your backup they would all be backed up in the event of a catastrophe.
Use Time Machine to back up or restore your Mac - Apple Support
you could also duplicate the folder with Carbon Copy Cloner, a 3rd party solution.
https://bombich.com/
if you have photos outside of your external drive the photos themselves are generally keep in
macintosh HD/Users/[your account]/Pictures
you can drag and drop the photos or folders into a new HD provided the drive is not formatted NTFS for Windows unless you have 3rd party software to provide read/write in that format. Other mac formats and window FAT32 and EXFat which are popular with off the shelf external drives will be supported.