Just an update aside on this.
It looks as if things still aren't quite right.
I'm seeing this error recorded regularly in the event log -
"The storage optimiser couldn't complete re-trim on CAPTURE (F:) because: This operation is not supported on this file system. (0x89000020)"
You might be interested to know that my drive F: is a FAT32 formatted hard disk drive, not an SSD!
So, even though it appears as an HDD in the list of drives, it looks as if the system might be still trying to TRIM it!
Windows 10 can't TRIM FAT32 SSDs, but it can defrag FAT32 HDDs.
I have another FAT32 drive on the system which is not throwing the error.
Cheers, Dave.