But when it ran in the read-only mode, it confirmed there were no errors. Yet just to be sure, I did re-run it with the fix flag:
C:\Windows\system32>chkdsk /F d:
The type of the file system is NTFS.
...
Windows has scanned the file system and found no problems.
No further action is required.
I went ahead and manually renamed the `LCU` folder to see if startup repair could go beyond that, but instead, it errored even faster with a new message.
TL;DR: My second system drive is 'a little' corrupt: files can be accessed, partition taken/used space information is correct, sectors are good but it just won't boot past Windows loading. Chkdsk, sfc etc. all good. The only error is seen in SRTTrail.txt (Startup repair) logs.
I now have 2 SSD...