After running a program for disk diagnostic made available by Seagate, my disk became blank. I was able to recover all the partitions with TestDisk, and their files are there just like before, except for one Ext4. TestDisk shows it, but the files are gone. Even without the files, I asked TestDisk to restore the partition. Not surprisingly, it doesn't mount.
I can see and even recover the files with R-Linux and PhotoRec, so I had hopes I could restore the filesystem, since the partition is (apparently) not severely damaged. But when I try to use alternative superblocks to repair the partition, none of them work (e2fsck: Bad magic number in super-block while trying to open /dev/sdb6. The superblock could not be read or does not describe a valid ext2/ext3/ext4 filesystem.) And when I check the superblock backups with TestDisk, it gives only "superblock 0, blocksize=4096 []".
I would like to understand what could have caused this. If all the files are intact, why could the filesystem be lost?
