Main
256GB SSD—for OS & programs only, small size makes the monthly disk image under 30 minutes, and easy to clone if I need to transfer to another PC.
1TB SSD—for games, where my replays are permanently installed. Others I install to play, then uninstall.
4TB HDD—for files. I use my PC for work & play, so I have a lot of documents built up over the years—but only half full.
4TB External USB—to backup the HDD and keep some archives of old stuff, plus a few 256 SSD images.
Server—I have a server share for our biz, and occasionally upload a backup of our most important documents there—painful with upload speeds. I also have a bunch of TBs on OneNote—part of MS 365—which I've used very little, but probably will/should.
Others
I also have an external floppy to recover some very old family media, and a 'toaster' for simple quick examination of various old drives and transfer of their contents if necessary. Then there's the 64GB USB plugged into our router, which serves as our network drive for swapping work files and keeping the 'official' versions of those we both work on. A few other USB flash drives for whatever.
This is a good setup for me, which I will likely repeat in my next build in a couple of years. Small C: drive is easy to manage properly, and contains only the essentials for my daily use. Games are an easy reinstall, OS & programs not so.
HDD Note
This is fine for files which aren't in frequent use. Until SSD prices/MB become similar, save money by using HDDs for large media—video, music, images etc.