There was an element of redundancy in parallel supplies where 2 (or even 1) could keep all 3 little PCs running.
It sounds like my mileage may vary and there are many a blue smoke trap. With the 3 little PCs combining to about £600 I'd rather not test the mileage in the theories. I have personal experience of "wayward" DC currents and just how far they will go to find the lowest resistance path back to their electron source ... which tends to be the one without a current sense shunt in it.
On the application side. Honestly only 1 of these drives is regularly called for. The "non-personal" media. Movies, music, TV shows, etc. etc. It's just too bulky to SSD and while stuff I download sits on SSD drives for a while, then they get cleaned out, they get dumped on to the HDDs. Pulling back a series or two if I'm watching is cumbersome. None of this is backed up. It's a single drive. It has suffered bit rot over the years and some loss, but it's been routinely transfered up to newer drives every few years. It is all technically replaceable.
The personal media drive is more of a long term store. When I need to use much of the media in there, working with videos does need to be transferred to a SSD local drive. It can be left offline for weeks. The "guts" of this are backed up, infrequently, manually.
Backups are in the form of ZRAID trio of 2Tb drives providing 4Tb of backup space with 1 drive loss tolerance. These and the box they are connected to are offline and only WOL'd when I want to sync backups to it, once a week for the routine ones. Online system drives are dual M.2 in the case of the larger server. Single SSD or M.2 in the case of the low power ones.
All the virtualised storage and VMs/Containers are backed up, 3 daily, 2 weekly, 1 monthly for 90% of virtual disks. All of these, plus config snapshots of all virtualisation servers is synced to those offline backup disks weekly, held locally on each node until then. The plan was to automate it to be once a day, but ... life/time you know the drill.
The virtualised stuff is so easy to deal with, backing up, migrating, etc. It's the physcial bulk storage which is the burden to solve. Hopefully in a few more years 10+Tb of SSD storage will be affordable.
I'm going to take the plunge and put an HDD on a Zimaboard without any PSU modifications and see how it performs.
The idea being... if I need to access that drive I can make it a simple "WOL" command, wait a minute, work away. I can (time permitting) even setup a "lights out" schedule for it. Meaning if the disks are spun down for longer than 30 minutes, shut the box down completely. Even without this I would be forgetting to turn off a 6W PC with a 5W drive, rather than my current situation of running the 100W server 24 hours because I didn't see it's blue lights on.