It is strange that you got different results from fsck and when running badblocls directly
I'm befuddled myself and have no explanation for the disparity.
Have you run smartctl on that device to see what it has to say as far as recorded errors and its overall health assessment
If you're that unsure about that disk then perhaps it's not a good idea to use it for backups.
Smartctrl reports "Disk is OK, 8 bad sectors (39° C / 102° F)".
I still think that one result might be inodes while the other is blocks. I remember, when I ran the fsck, it found the bad
blocks at around 80%. The drive has 1,953,513,559 blocks, concluding that block 337,905,103 would be fairly
at the beginning and not at 80%.
I also learnt that the SATA-USB adapter I've been using is not to be trusted. When I started copying data to the drive, I had
random checksum failures. So I attached the drive directly to the internal SATA bus and managed to copy over like 800GBs
which all read back fine according to md5sum.
Of course the drive is still faulty. I removed it from my system like 5 years ago for that reason. And of course you're right,
that I shouldn't use it for backups. However, I'm not going to backup anything important to that drive, mostly old video
files (tv-shows from my sat receiver) that I've already watched and most likely won't watch again, or stuff that's already
backed up on another drive and on bd-r.
I still haven't found any good way to store large amounts of data, bd-rs are too small, the media is fairly expensive and
can deteriorate, external drives can fail and aren't cheap either. So I backup important data using double backups
with both and on a NAS drive. The only reason why I dug out that old disk again, is because the space was getting low
and the alternative would have been to delete stuff.