I've just moved 262GB of photos to a new external hard drive (1 TB), freeing up my MacBook Pro, 1 year old, which was dangerously low on space. I first backed them up on an existing external hard drive (also 1 TB). But now I see that this hard drive is getting full and Time Machine has started deleting older backups. That's ok. Currently the backups go to Jan 2015. But I need to keep the backup dated 2015.06.11 because that has all the photos I just deleted from the computer: these images are now on my new HD called "Photos" (and not formatted for Time Machine) and backed up in 2015.06.11 on the original ext. hard drive. So for now I'm okay, but in order to keep 2015.06.11 will I have to replace my backup ext. HD? Or is there some way I can tell Time Machine not to delete that one backup? Thanks.
Some advice. I always back up my photos separately from Time Machine. You can save your images in a separate folder on the Time Machine drive and Time Machine will not overwrite them (used to do this with Apertures backup vault).
Also, depending on the space on your Time Machine drive, you could use time machine to backup your external Photos drive as as well.
I would recommend that you have your photo's in at least two locations (which you currently have; however, Time Machine will eventually overwrite old data that is no longer on you hard drive if it needs the space).
I recommend that you get a third drive (2-4 times the size of your non-Time Machine volumes).
Your current Time Machine configuration doesn't really allow you to keep a snapshot of the backup dated 2015.06.11 (~900 GB) because none of your drives have enough free space to store it.
After installing a 3rd external drive (with at least 2TB, the more, the better) and promoting it as your new Time Machine backup drive, you will be able to properly backup your internal drive as well as your external "Photos" drive in one Time Machine backup. Source.