my My Book Studio II used to be configured in RAID 1 (mirror) using the WD Raid Manager software long ago. Today it showed up as RAID 0 (capacity), and OS X prompted me to initialize it. (I didn’t)
what can I do that gives the the highest chance of making the data accessible?
A DataRescue “quick scan” tells me it’s not an HFS-formatted disk. A “deep scan” will probably do well on files, but will probably lose the directory structure.
I am reluctant to wipe and restore from backup because the El Capitan 10.11.5 update appears to have caused the system to stop recognizing the OS X “Concatenated Disk Set” where my most complete backup resides. It shows up on some boots, but not others. I set that aside, bought bigger drives and was in the process of starting over when this occurred.
The good news is that the backup concatenated disk set did mount. I plugged it in and opened disk utility, which spun the beachball overnight. the next day it offered to mount it in read-only mode, so I grabbed a copy of the data onto a new drive. Problem solved.
Out of curiosity, I tried DataRescue on the My Book Studio II and it was not useful. While in RAID 0, it identified just under 1% of the files (the whole drive was JPG or MV4). Changing the RAID (risking losing everything) back to RAID 1, DataRescue identified just over 1%.
Backups remain your best bet, but Trancer is right, professionals would otherwise be required.