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I have a 1.8TB Disk that was previously formatted EXT3 with a very complex folder structure and 1.4 TB of critical files.

The disk was plugged into a windows server and assigned as a backup disk, which means that a new NTFS partion table was written

no data was ever written to the new partition scheme

my question is this, can I recover the previous partition scheme with TestDisk to access the files, or should I just skip to trying to recover the files with Photorec, and abandon my Folder structure (which has significant importance

It should be possible to recover the ext3 partition using testdisk, Analyse, Quick Search, Stop, Deeper Search. When the partition is found, you can choose Stop.
Set this Linux partition as P(rimary). On next screen, choose Write, confirm. In Advanced, select the partition and choose Superblock.
After a reboot, use fsck.ext3 with the values displayed when in "Superblock" to try to fix the filesystem.
Good luck