Corrupted Storage Spaces Protective Partition

Question

I am running Windows 8.1 pro build 9600
In addition to the boot and data drive, I had two internal 2TB drives set up to use all their available space as a mirrored Storage Space Protective Partition, which, before the problem below occurred, contained about 1600GB of -- you guessed it -- largely
un-backed-up data.
I thought that if one drive fails, the mirror will act as backup, while stupidly forgetting the possibility of file corruption or ransomeware.
So, while the computer was in 'sleep' mode overnight, there was a power cut, and when I tried to reboot, Windows automatic repair kicked in and said it could not fix the system.  I managed to boot to command prompt, and diskpart reported both offending
1863GB GPT disks online, with no free space
MOUNTVOL reported that the offending volumes had *** NO MOUNT POINTS ***.
CHKDSK appears unable to work on an unmounted partition.
I could try EaseUS pro data recovery, but from previous experience this will take anything up to 3 or 4 full days, and will not repair the damaged mirror.  However, I'm not sufficiently familiar with DISKPART to know if, for example, the RECOVER
option might be helpful, or if it might further mess up the file system, in which case a commercial data recovery like EaseUS may have more difficulty in sorting out the mess.
I'd be most grateful for some ideas about what to do next...

Answer 1

Thank you for your reply.  Since my fist post, I have managed to start the PC from a duplicate boot partition set that I made after upgrading to 8.1.
Disk manager shows the disks as present, healthy, and marked as Storage pool.
However, all options are greyed out so I cannot assign a drive letter, and as before, MOUNTVOL reports
*** NO MOUNT POINTS ***.
The problem with the recovery apps I've tried so far is that after scanning either drive, they find the partition, but classify it as UNKNOWN.  If I run the same apps on other disks they can recognise all the usual FAT, FAT32, NTFS, ext2, ext3, ext4 etc.,
but they don't seem to know about Storage Spaces Protective Partitions.
Do you know of some recovery app that understands this type of partition?
I know I can try getting the recovery app to read it as a RAW partition, but the problem there is that from previous experience I guess it will take 20 hours or so to run, and then not be able to build a useful directory tree.
There are probably 200,000 or so files on the partition, of which I only need to recover one folder in which there are about 400 files not otherwise backed up.