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The problem

I recently got the following error trying to mount a ext3 partition on an old hard disk:

couldn't mount because of unsupported optional features (2000200)

Trying to mount it from the terminal says:

mount: wrong fs type, bad option, bad superblock on /dev/sdc4, missing codepage or helper program, or other error. In some cases useful info is found in syslog - try dmesg | tail or so

And dmesg | tail output:

couldn't mount because of unsupported optional features (2000200)

I then tried fsck with no success. It kept saying "If the device is valid and it really contains an ext2 filesystem (and not swap or ufs or something else), then the superblock is corrupt, and you might try running e2fsck with an alternate superblock: e2fsck -b 8193 "

Running e2fsck -b 8193 as suggested, had almost the same output.

The solution

But finally, I have found a way to solve it. If you too have this problem, here is what you need to do:

1. Run the following command in a terminal:
mke2fs -n /dev/sdc4

sdc4 was my faulty partition so replace sdc4 with the partition you are trying to fix. You should see something like this:

mke2fs 1.41.9 (22-Aug-2009)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
3391488 inodes, 13548819 blocks
677440 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=0
414 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
4096000, 7962624, 11239424

Notice the blocks in bold. We will use each one until fsck will work. Start with the first (32768) and keep going until one works:
fsck -y -b 163840 /dev/sdc4

Also replace /dev/sdc4 with your partition.

2. Once you have found block that works, run fsck to try and fix it:
fsck -c /dev/sdc4

Once again, remember to replace sdc4 with your partition. This will take a long time, but your partition should fixed.