It seems my main disk was dying and eventually, Windows displayed a message to immediately reboot into recovery, make a full backup and replace the HDD.
I turned the computer off and plugged the HDD into another computer. I was able to still see the disk and navigate the file structure on both partitions (I had one for the OS and another for Data). I actually didn't try to open any file, I just browsed a few files to see if I was able to at least see what was in there.
I then downloaded and booted up SeaTools. I first ran the short test, it failed immediately. The HDD was definitely dying... I then ran the long test through the night to see if it could mark all the bad sectors so that I could copy everything else to another drive.
It showed a lot of errors and attempted to repair a lot of them but I think it only made the problem worse. Because now I can't even browser the files. Actually, Windows (on the other computer, of course) takes ages to boot with this dying disk connected. It boots quickly if it's not connected.
I first tried a Live CD of Linux Mint and it also failed to boot. I then tried a Live CD based on Knoppix and I was now able to boot, but couldn't do anything. The HDD does not appear for me to browse (other HDDs do) and all tools available report lots of I/O errors, damaged partition table and boot sectors, etc...
It's not the end of the world if I'm not able to access the data on this particular HDD, my important data was backed up on another disk. But it would be nice if I could still copy some data off of it, or at least try to browse the file system and see what was there.
Any ideas on how can I fix this since I was able to browse the HDD before running the SeaTools long test?