What's the current amount of space you're talking about? What product are you currently using to do backups? Are you using any cloud backup services currently? Are you a Lightroom user, or other products?
I know from experience how important having some sort of structure to your photos is - and it never gets easier to 'clean up' than now.
If you can swing it, what would consolidating your photos on to a single drive require? 5tb? 10tb? more?? Snag 2 USB3 drives (8tb?), copy all your current drives to 1, and get to work cleaning (and sync the updates to the second one). Programs that find dupes are fun, but deciding on a consistent method of naming and organizing will help you going forward.
Concerned how to split out different works? This is where keywording comes into play, or naming of directories.
All good points. If this question was directed at me, the original poster, then... I'm guilty of not being organized. Still photography is not my main business, or really, a business at all. So, I've got a lifetime of photographs without much guide to sort them, save maybe, when they were made.
I started to place them all on one 3tb drive and to make copies of this drive. But, I was recently traveling, and there are a whole bunch of new photographs on my laptop, and one back-up drive. And so, I no longer have a master drive with all the photographs. Right now, I really just want to get all the photographs again on a single drive and then, back that drive up. After this, all new photographs will go to the master drive and I want to be able to have software to compare the master drive and the other back-ups, for new photographs, and just back up those, without copying the entire drive.
I think that cloud back up for 4 tb of photographs might be too difficult and expensive. But if you have a suggestion for a cloud back up solution, I will look into that also.
After I get 3 drives with copies of all the photographs, I can then contemplate organizing them. There are only about 500 "finished works", and maybe 2000 RAW files here. Not, 10's of thousands as many have