We have just completed a complex data recovery, where a Mac system had been inadvertently overwritten with Windows. The Mac drive originally had over 500GB of data, so we expected to get most of it back, we just didn’t know how good the structure would be.
It helps to visualise the layout of the data on the disk. before it was overwritten, the data would have looked something like this:
Although the fresh Windows system is much smaller than the original data, it prevents you from seeing any of that old Mac data.
Once we made copies of the drive, we were able to reconstruct the missing parts of the Mac data, and could see all the original files and folders, with their original structure.
Luckily nobody had tried to fix the problem with this drive. Often the fixes people attempt are worse to recover from than the original problems.