HELP--Storage Space Drive Suddenly Missing, Files Inaccessible

TL:DNR - The issue:
A 2-disk mirrored Storage Space is suddenly missing from explorer, “get-disk” and disk manager; so I cannot access 2.5TB of incredibly important files. The disks still show in storage space management, “get-physicaldisk” and “get-storagepool”.

The system:
Windows 10 Pro, updated about weekly. Disks both connected via SATA.

Additional info on current state:
While the disks DO show in storage space management, the storage space itself no longer has a drive letter assigned to it and will not let me add one. One of the drives says “OK” under status, the other says “Warning – Error; consider replacing”. Hitting reset next to the unhealthy disk results in “Can’t reset the drive… The specified volume is offline (0xC038004B)”. The drive is not, however, disconnected—if I remove that drive, it’s status changes from “Warning – Error; consider replacing” to just “Warning”, which I believe is the standard status for a missing disk.
Rebooting with one or the other disks disconnected changes nothing except which drives show connected in the manage storage space UI. Still no drive showing in explorer in any situation.
The disks both show in “get-physicaldisk”, one with the status of OK, the other with the status of Split. “reset-physicaldisk” in PowerShell also results in “Can’t reset the drive”.

Disk Management view: https://i.imgur.com/UaAr5MM.jpg
Storage Space Management view: https://i.imgur.com/lS6TkAo.jpg
View of various PowerShell physicaldisk commands: https://i.imgur.com/IJpVrw9.jpg

How we got here:
Everything was working fine, and I was attempting to copy a file from my laptop to the drive over my LAN. The transfer was going incredibly slow (like, 355kb/s), so I hit the cancel button on the transfer. It wasn’t canceling, so I just closed the transfer dialog window. A couple minutes later, I went to reboot the computer in safe mode, and it had a Windows Update to apply during the reboot. There was no dialog about force-closing anything before reboot. After the PC turned off and back on, and started doing the post-reboot windows update portion, it said the update failed, reverting back to yada yada. Then it continued applying the update (which may in fact have just been the dialog for it continuing to revert back, I don’t know). PC was up to date as of about a week ago at the latest, so it wasn’t a major update or anything. When it continued to windows and I opened explorer, the drive was gone.

Additional relevant info:
These drives are about a month old, and this happened once before a couple days after I received them when I had temporarily taken a drive to another system to transfer data. When I brought it back to the PC in question this same exact thing happened. I couldn’t find a solution then, so I deleted the storage space, created a new one, selecting the two eligible drives now that they were free of a storage space. I still had my data stored on other drives at that time so doing a wipe and restart wasn’t a big deal. After I was done transferring my data, I gave the drives a week with no issues before I deleted the data from the other source disks, convinced my mirrored redundancy would protect from any issues.
When this issue occurred, I didn’t believe it had anything to do with hardware. I know Storage Spaces can be finicky between versions of Windows, and since I was connecting and transferring data between multiple systems that were all in the process of being brought up to the latest Windows 10 version, I chalked it up to my splitting the drives between systems temporarily, which was not part of the catalyst this time.

Thank you so much for the response, please see below:

dalchina said:

Three separate points:
a. As you may know, feature updates (upgrades) seem to be fundamentally bad news for storage space from what little I've seen. Perhaps that has improved with 1809- I don't know.

b. It's definitely worth while having disk monitoring software running to detect early indications of failure. E.g. HD Tune, Crystal Diskinfo, configured to generate alerts.

c. Try installing Hard Disk Sentinel (trial). Best text indication of what's going on - right there on the GUI, succinct.

Please post a screenshot of your Update History
To post a screenshot please use the Insert Image icon above your post. Thanks.

Which Windows build are you using?
This can be really important, and most people don't mention it or include it in their specs.
Windows key + R, winver

a) I've been using storage spaces on this PC for about 2 years and while there have sometimes been compatibility issues when using a drive on a different computer, updating said computer has always fixed the issue. After this experience I'm pretty sure I'll never be using storage spaces again (and possibly never using Windows again after experiencing their support for about 5 hours today).

b & c) Thank you for the advice. While I don't believe it would've helped avoid this particular problem (as I check my disks regularly with GSmartControl and this new drive has given no indications of physical failure), in the future I'll use a monitor.

Update History:

Windows build: 17134.472

Thank you again for your help and candor. You don't need to read the below as it doesn't really add anything to the situation, but I really, really need to vent for a moment.

*begin rant* Chatted with Windows tech support, after an hour of them controlling my desktop and looking at what felt like everything except the issue, was given a phone number to Windows commercial assistance. Spent a total of 4 hours on the phone with them being friendly as can be, having explained the issues no fewer than 15 times, watched them tinker with USB drivers and all things not related to the issue, been hung up on 3 times during transfers to the "professional" team, who as it turns out, were hanging up on me because they don't enjoy giving the news to people being transferred from the "commercial" team that they will need $500 to fix the problem.

So, I spend $400 on new drives (that I like, basically had to create a go-fund-me for my friends and family to gather the money), choose a Windows software solution to create a 2-way mirrored redundant storage space to protect my data, and then that software, rather than preventing an issue, creates the issue that renders all the data on both mirrored drives inaccessible. If I had just used the drives out of the box with no redundancy, I'd've likely had years of usage out of them with no issues, whereas now, less than a month old, they've already lost 2.5TB of data. *end rant*

Thank you again for anything you can do to help me recover the data. The disks are there, they show that they have data on them in storage spaces. I just need to actually be able to access that virtual drive that now has no drive letter assigned to it and doesn't show up in disk management.