OK, TChad, here's my setup. My desktop has 2 HDs. The W10 OS is on C:, 500GB. The D: drive, 4TB, is simply my storage drive, so that, if my C: gets messed up--and it did--all my really valuable files (15 or more years worth) would not be lost or compromised too.
That's all in an upstairs office. I'm downstairs in the evening and to avoid having to go upstairs for whatever I need computer-wise--I'm 79--I work on my laptop.
BTW, this afternoon, I did some messing with stuff and got to where now the laptop sees the desktop as well as the D: drive, but I still cannot get to the folders there. In other words, I've made a little progress.
This all sounds like your problem is not with local disk access, but with your network, which I gather is probably Microsoft's built-in peer-to-peer Windows networking. The first solution that comes to mind is that you could reinstall that network from scratch. An alternative might be for you to install remote control software such as Anydesk, TightVNC, or the newer program RustDesk. That way you could remotely edit files on your upstairs computer from your downstairs computer, or vice versa.
Good luck.