AO,
Have you tried the process after a clean boot?
I have 3 different external drives and for some reason my computer declined to read them. I have now done that process on each and it worked each time. Do try once more.
There are one or two more things I need to do which is to ensure that the different drives could be read together, ie different drive letters. I have a card reader that uses 4 or 5 drive letters and need to ensure that it remains compatible.
I don't know why this happened but as I am not alone one begins to suspect the dreaded security updates.