Need to do this for photos and videos

While I'm not that concerned about JMRI and other model train related stuff since all that can be reprogrammed, I do need to do this with my photos and videos.  Problem is I've got everything spread out on three different computers all full to the brim.  My plan in January is to build three DIY external hardrive storage (the sort that looks like a desktop and holds many TB of data) and keep one of the hard drives in a storage unit, brought out every month for an update.  The other two are kept in the house with one updated every time I change something and the other updated about once a week or so.

Fire?

That's what I see as the greatest danger. I would think that 99% of us keep even the back up in the same house as the main source. I mentioned in other forums/threads that I use flash drives. Multiples. So at least 3 or 4 different places.

What I need to ask is: How do you get an "automatic back up"? I save a file on the HD, and then save it on a back up flash, and then another flash drive or two more. So, it takes some time and effort. But as you've stated, a necessity in today's world. And just FYI. I've had only one flash drive fail in 20 years. FYI: 99% of my flash drives are PNY, a quality brand. The one that failed was a no name brand. 2 of my PNY's have been working for those 20 years. I admit to being a little paranoid about saving anything in the cloud.

So ? I should buy a separate SSD hard drive, ? of what 2TB? 4TB? and as I asked above, how do you make it Automatic?

automatic backups

I'm a Mac guy, so I know that well. Time Machine is a part of MacOS, and you can go to the OS Settings app and you'll find a Time Machine icon there. I won't repeat the instructions but basically you're nominating a target on which to put the backups, usually an HDD although SSDs work too. It saves hourly backups for 24 hours, then daily backups for the past month. Weekly backups are kept for as long as there's space on the drive. When the drive fills up, the oldest ones are deleted to make room for new ones. I generally target having a backup disk twice the size of the live disk, so that the history goes back a good long time. I'm in a little different situation than most people, but I have about 9TB of live data (400k pictures, many of which are 45 megapixels), and my two backup drives are 16TB. At the moment I can go back to last January on either one of them, so a bit over ten months worth.

Windows 10 (and, I presume Windows 11) has a similar arrangement. Go to Start -> Control Panel -> maintenance Backup and Restore, and then select Set Up Backup. I've done this a couple of times for other RR club members but I can't remember the details. But basically it's similar to Time Machine above in principle. It makes occasional backups and keeps them as long as it can.

The key to both is that it's automatic. Once you set it up, the system takes care of copying files, you don't have to.

Linux has this capability too, I can discuss if necessary.

You absolutely want a physically separate drive, in case something happens to the original one. For size, see above. If your data is now 1TB (or even just a 1TB drive) I'd go for 2TB, although it isn't necessary to go that far. OTOH, if your data is 500GB or 1TB, the next increment isn't that much more money in absolute terms, especially for HDDs. It's when you get up into the bigger sizes like my 16TB that 2x gets a bit expensive.

I'd also suggest using HDDs for backup, because SSDs (all of the affordable ones) have wear limitations. The wear limitations are much more restrictive on thumb drives than on other non-enterprise SSDs, but they all have limits.

I've had more than that fail, but I've had way more SSDs than you have.

While I'm not that concerned

That may be true, but it's a hassle. I have buildings drawn for a laser cutter, a few 3D parts for the 3D printer, my entire rolling stock roster, and probably a dozen other model railroad things, in addition to JMRI stuff. The thing about this stuff is that backing it all up costs nearly nothing. All of this stuff for me is about 2 GB (not 2TB or 2000GB). Once you set up automatic backup, you will never remember that it's there... until you need it, in which case I'm pretty sure you will be grateful for it. In my case in addition to DecoderPro stuff, I have the entire layout in a panel, with all of the turnouts, block detection sensors, signals and routes... in addition to most of a timetable of trains for the computerized dispatcher to run. It would take me weeks to reproduce it.