Non destructively Convert offline dynamic disk to basic disk using testdisk tool

Solution 1:

Some moron, likely previously employed, at my workplace installed a second 500GB hard drive in two systems and set it to Dynamic.

The first time I encountered this it bit me after I reimaged the system. 500GB of GIS data needed by an engineer ...

I followed this -analysed the drive, picked a partition with P and used the "c" option it mentions below to copy all files off to an external USB 2.0 disk. It took about a week to copy all the data, but in the process the original drive was untouched. Basically testdisk lets you traverse the directories and copy off data without modifying the disk.