Secondary Backing Up of Photos Outside of Time Machine

I don't delete photos from the memory stick from my camera so I have a secondary backup of photos from my camera but I can't do that with the Iphone 5 and 6 that I have. With 99% of all my photo's being taken on the Iphone 6, i want a way to back them up other than on Time Machine.

I have a external hard drive that I use to run Time Machine with but I have more lots of photo's that I don't need on my 2008 Mac laptop. I probably don't have enough room on the 250 GB SATA Disk internal hard drive to hold all my photo's.

I have burned some of the photo's on DVD, is that good enough for long term storage?

Can I somehow use my external hard drive to use it to store files that are not on my Mac's hard drive and still use that hard drive for Time Machine?
I did a Time Machine of all the photo's last time I upgraded to Yosemite but I can't find any of the photo's from the that saved file. The photo's are just the photo's that are on my laptop current hard drive.

I am planning on doing a clean install of El Capitan from Yosemite as El Capitan is the last operating system my 2008 can handle if that makes any difference.

I've been looking at the same issue the and one of things I'm considering is a storage expansion card which inserts into the SD card slot. You will first have to get your photos from the phones to your Mac. By default, Photos is set to open automatically as soon as you plug your iPhone into your Mac. It will show all the photos that are stored on your iPhone’s camera roll, so you click ‘Import All New Photos’, or select individual shots, to import.

https://www.amazon.co.uk/gp/product/B00WGARJGU/

The issue with all storage devices, especially DVD, is how long will the correct technology be available from which to access the material. Whether for cheapness, weight or greed (buy their separate SuperDrive...), Apple no longer include a DVD drive on any Macbooks. Even I now download music rather than buy the DVD.

External DVD drives will be around for a long time to come. Believe it or not, you can still buy an external USB floppy drive to read your old 3.5" disks that were made back in the last century. Keeping photos backed up on a DVD is actually safer than keeping them on a flash drive or an external hard drive. Another idea is to buy space in the cloud and store your photos there - in other words, off site backup. Just make sure you choose one that is secure and will be around for awhile.