I keep getting, "We couldn’t update the reserved partition" when I try to install Win10. I downloaded it onto a flash drive and then copied that to the desktop. I also tried it from the flash drive itself.

The reserve partition seems to be a common problem with upgrades. The directions below are fairly common in trying to solve this problem. They refer to Win8.1, but the same thing can be found for Win10. The thing is, I don't have that 100MB boot partition. As you can see, my boot partition is about 464GB.

Are these directions telling me to rename C: to Y:? I'm open to suggestions.

From the web page -

"1. Press Win + R, type diskmgmt.msc and click OK.
Right click the boot partition listed at 100MB in a map, select Change drive letters and paths > Add, and choose Y: for it.
2. Press Win + X, select Command Prompt (admin), and run the commands one by one:

Y:
takeown /f . /r /d y
icacls . /grant administrators:F /t
attrib -h -s -r bootmgr

3.Open File explorer, go to Y:\Boot, delete all except en-US. languages.
4.Run command again:
chkdsk Y: /F /X /sdcleanup /L:5000

5. Now you can start the 8.1 installation. After upgrade has been done, go back to remove the drive letter."

I'm confused, Jerry. Have you tried to follow those instructions?

Can you get to the part where you add the drive letter Y: to the restore partition? (Can you complete steps 1 and 2?)

If so, step 3 might be irrelevant to you. (It was to me.) Step 4, chkdsk, is important because it fixes the directory structure of the Y: drive. (In my case, the Recycle Bin had bad links in it.) One of those parameters, I think it was /sdcleanup, might not be part of your system and can be safely omitted.

If you can get past chkdsk and the Y: drive has at least 50 Mb free, you should be good to go.

This is a little bit of a long shot, but try swapping the SATA cables on your two drives so your SSD is Drive 0 and your 4TB is Drive 1.

I've run into Windows install having trouble with the EFI and recovery partitions if they weren't on "drive 0" in the past. You may have to adjust your boot drive in EFI/BIOS to accommodate the change.

-OR-

The "boot partition" those instructions are referring too would be your 750MB recovery partition that you'd mount with a drive letter. There's a good chance it's full given it's small size. You may need to increase the size of it to a few GB as well.