I have tried to fix the bad sectors of an external HDD following the advice of user228505, that is, installing smartctl
sudo apt-get install smartmontools
sudo smartctl --all /dev/sdc2
sdc2 is where my hdd is) and then I've obtained the following error message:
Long (extended) offline self test failed [unsupported scsi opcode]
Does anyone know how I should proceed? Is this because my external hdd might be too old? Thank you!