Ran it with the /f option. No errors encountered. I just bought the h120 and it was shipped, but packed really securely. Do you think that this "hardware failure" might've been caused by the transportation?
Maybe, or maybe the disk is dying because of its age. I really can't say. IMO it's possible but not necessarily the cause.
That's not an option, because I need both, the h120 and my h140.
You can of course switch the HDDs back once the test is completed

But I understand that you might not want to disassemble a working device you want to use.
Well basically I want to remove the bootloader and according to the wiki article I need to "simply flash an unpatched Iriver firmware."
I have no idea how to do this and would be extremely thankful if somebody could brake that down step by step.
Just flash an unmodified firmware the same way you flashed the Rockbox-modified firmware. Since installing the bootloader requires an unmodified firmware file to get patched you should have that file already, if not you might want to have a look at
http://rasher.dk/rockbox/iriver-firmwares/What happens if I format the hdd with Windows 7 after I've uninstalled the bootloader?
The device will work as if didn't have Rockbox installed. Of course, if you are experiencing a hardware failure that will persist. Make sure to format the drive as FAT32 since both Rockbox and the Iriver firmware won't work with NTFS.
I want to do a fresh&clean install of rockbox or maybe first upgrade to the 1.66 E iriver firmware, then uninstall the bootloader and after that format the iriver's hdd? Would that be correct or am I doing something wrong?
You can do that, but IMO this is unnecessarily complex. The Rockbox bootloader is patched into the Iriver firmware which runs from flash memory. Thus, a broken harddisk won't have any effect on the bootloader working or not -- if you have flashed it successfully there's no need to replace it (unless you're running a very old bootloader).
Also, the h120 uses a hardware bridge for accessing the disk, the firmware only enables the bridge when USB is connected. Therefore it makes no difference if you're using the Iriver firmware USB mode, Rockbox bootloader USB mode or Rockbox USB mode. If you want to use Rockbox on the device I'd simply use Rockbox bootloader USB mode for troubleshooting the disk
P.S: Bluebrother, how do you interpret the CrystalDiskInfo? Is the drive faulty/damaged? I know that CDI is not 100% accurate, but still it's something I guess...
I can't see anything obvious pointing to a specific problem here, but I'm not good at reading such output.