I did accidentally override the first part of a partition. Please note, the GPT partition table is completely intact. No damage done there.
That partition was as large as possible. Since I only have another same-sized disk (but not larger), I created a disk-image of the partition only.
I try to run testdisk on that image but all available options seem to include rescuing a partition table. That answer on my last question seems do imply that testdisk can very much recover partially damaged file systems – exactly what I need.
So, how do I run tesdisk on a partition image only (I know the file system type, it's ext4) with the purpose of recovering whatever data is still there.
I already run foremost. The result is nice and all, but my most valuable data is a) a data.db sqlite database (that's a not-supported file type as far as I'm aware) and b) a .thunderbird folder (the folder-structure is quit important for that one)