Please don’t suggest fiber runs, as the cable is already run and I don’t plan on redoing it.

You said you have conduit, right? Running fiber would literally be as easy as: unplug Cat6 -> tape fiber to end of Cat6 -> pull on other end of Cat6 -> done. If that takes you 5 minutes you’re pulling too slow. Then, just throw on a couple of SFP adapters and you’re solid. No more worries about ground loops, greatly reduced chance of a lightning strike frying equipment on both ends. I think that’s worth a few minutes spent pulling fiber.

Mail server or anything using RDP.

For one thing, everyone’s going to tell you not to directly bury cable on a run like that – bury a conduit, run it into the buildings on both ends, then pull the cable through it. And use metal conduit.

For another thing, everyone’s going to tell you that you should use fiber for the run, but since there isn’t power at the other end, that will be a problem unless you can also run power out there (but use a separate conduit for that).

That said, I’ve seen ethernet surge protectors installed on lines coming down from poles where long-distance WiFi transceivers were mounted, clamped to an appropriate ground. If you use buried metal conduit, it’ll take care of most of the shielding and grounding issues, and if you don’t have anything in the way of an exposed outdoor pole, you shouldn’t have too much issue with direct lightning strikes (in my opinion).

While you’re burying conduit for a couple of ethernet runs, make it large enough for 4-6 runs, and then bury another one maybe a foot away (but check with reputable electricians) for a future electrical upgrade to the shed. You might not be able to do anything now, but at least you’ll be ready to just push some electrical wiring through in the future, and you won’t have to dig up your ground again.

I’ve got 9 of the 8TB IronWolfs in my server, and they’ve been great. The 3 oldest are 4 years old now, and I’ve grown the size of my array as I could over time, and switched from RAID 5 to RAID 6 for the dual redundancy. Speed is not important in my environment, so I can’t speak to that vs WD or Hitachi, but they’ve been fine for me.