Another setback, rather major, being the connecting of the external drive... eventually not only completely blew out a USB port, but damaged the entire hub it was connected to wnough to cause frequent I/O errors.

It would seem there's quite a bit to be considered in DC power distribution.

Imagine a 5 foot long power cable to the computer and another to the external hard drives, both connected to my 12V car battery... say that's 1A@12V for a lowly PiZero+display and 3A@12V for two hard drives... say there's one ohm in those cables... now "ground" at the computer is 1V above battery-ground, and "ground" at the drives is at 3V! 

Now, imagine what happens when that USB cable is connected!

... worse, still, I did some falstad circuit simulations today to see about the best place to put a ground when using buck converters... and I was reminded of another factor I hadn't been considering; powering up a buck converter causes a huge power surge.
So, Usb drive has a power switch, let's ground 'em through the USB cable *before* powering up.... compy-on, no biggy... maybe draws a little current through the usb shield/gnd wires, but certainly most through its power cable... now, switch on the 3A drive... heh. Huge surge spinning up, of course, but also huge surge charging the buck converter's input capacitor, huge surge charging up the inductor, huge surge charging up the output capacitor, and the motors aren't even spinning yet!

So... I replaced the hub, and *thankfully* that seems to have been the only damage.

But, I need to do some serious grounding thoughts before continuing.

...

Stupidly, I had forseen some of this; I was building a power-bank last year[?] which has numerous floating-output DC converters for pretty much *exactly* the purpose of preventing such things... but, ultimately, it was such a hassle, and I guess I *really* needed to get some actual compy-time in... feel like I was actually progressing toward my original goals... and, well, I think I was more cautious about it in the beginning, then kinda forgot about it... and I guess it finally came 'round to bite me in the ass.

...

Thankfully, again, I think the only damage was to the hub. Although, I ran some extensive tests [copying a large file 100 times] and found that, in fact, *somehow*, despite *numerous* checksums and retry-mechanisms all along the process [sending the command over USB to read the sector, reading the sector, transferring it over USB, transferring it *back* over usb, writing the sector, nevermind the process reading it back for comparison], somehow it did the job once or twice *without* 'cp' complaining, and *with* erroneous data! I don't understand how that's possible! There should've been a checksum failure somewhere, wherever that was it should've automatically reattempted until there wasn't, or /errored/... but no error, and still bad data. HUH.

So, now, no idea whether all those millions of files transferred to my backup drive, and from there to my main drive, are accurate... am I really going to 'diff' them all? Especially after the reorganizing? UGH. I was *SO* ready to wipe those original drives so I could finally put this *years*-long process behind me.

...

And I still haven't figured out what to do about grounding, short of dangling a wire off each component and installing a bunch of friggin' banana-jacks in the van's wall.

...

And, I'd much rather be working on #The Artist--Printze, [surprised I didn't have grounding issues with that!] or finally getting to #Floppy-bird, or several others... but... not without a compy... a /properly-grounded/ compy...