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Please correct the size figures on the first two lines of your posting to avoid confusing other readers!
Did you remove the disk from its enclosure?
If yes, put it back in and rerun Testdisk (the drive should be in its initial setting).

If that does not yield a result you can

1. Run Photorec
2 ...

I made a backup of the 4TB external HDD, ran TestDisk Analyze Quick & Deeper.
There is no partition listed that matches the original ~3.7TB NTFS partition.

This problem originated because I was making a new LiveUSB, I ran fdisk -l, got my device list, then connected a storage device for the ISO, in ...