In terminal, I entered 1 diskutil list [enter]
2 diskutil verifyDisk disk3 [enter]
This forced the disk to mount. It was not even accessible to disk aid before... Phew... Done!
Now transferring all my data to a more reliable drive. Oh, dear. Thank you!!! After that, I will format the drive to a mac friendly format - not compatible formats....
To explore all the functions of diskutil, in terminal type