Lots of "Disk full" Errors, Disks Have Ample Space

Setup Background:
Recently, with the help of a handful of fine folks here, I moved all of my data files to a separate hard drive (D:\) and moved all program/OS files to a new, 10,000RPM drive (C:\) then used the original drive (identical to D:\) to set up RAID 1. After all that was setup and running, I finally sourced a TPM for my motherboard and enabled bitlocker on everything. Somewhere in the mix I had to use regedit to change the default locations for my user files from "C:\Users\user\Documents" to "D:\Users\user\Documents" and reinstall a few apps that got confused because of the switch (AutoCAD especially).

Here's what Disk Management has to say of everything:


The Infernal Problem:
I've been getting "Disk full" errors lately and I suspect it has something to do with the C:\ to D:\ regedit switch. As far as I can recall, I haven't received any errors from Windows itself, but right now the downloads bar in Chrome has three failed download attempts. Sometimes retrying the download will make it work, but I just did that and this is what I sometimes get after reattempting:



I canceled the first one, but the other two have just stopped altogether. I also got this error using P2P file sharing, reattempting usually gets it going.

In Summary:
I keep getting occasional Disk full errors from non-windows software when my disks have ample space. What's likely causing this?

The plot has thickened?

While writing up a summary of those changes I did for the thread it was under, I opened Windows Storage Spaces to find an error.

Under storage spaces it says "Warning, reduced resiliency, check the Physical drives section"
Under Physical drives section it says on my second drive "Error; retired add a drive then remove this drive"

I tried taking a screenshot of this, but now Paint won't let me save the image to D:\ (disk full) and if I try saving it to C:\ or even a flash drive, it says "Paint cannot save this file. Save was interrupted, so your file has not been saved." but it did create a blank .png file where I tried to save.

Could this be what's causing my disk full error on D:\?
If so, what kind of error could it be? SMART doesn't seem to be finding an errors because otherwise my computer would notify me during POST. I can't actually see the detailed SMART status (because I can't install the software to do so since my user temporary location is needed to install it and that's in D:\--that'll teach me to keep diagnostics software as install files on my NAS instead of keeping them installed on my desktop). wmic tells me that all of my drive statuses are OK.

So I assume this is related, but maybe it isn't. I have lots of 153 errors for Disk 2. There's also a 52 on Disk 3, which doesn't make sense because that's the storage space I thought? 0 is C, 1 and 2 physically make up 3 which is D:\. I also have 52 and 157 errors for disk 4, but that's the disk I use for hot-swap backup and I know it has a SMART error (which it says is critical but isn't).

I find it odd that one of my RAID drives would start failing within a couple of weeks of enabling RAID (I don't have that good of luck).

Obviously it looks like I need to replace a drive, but is this what's causing the disk full error?