Unrecoverable error after conversion to ext4

[Jonathan Hudson]

I (attempted to) converted a scratch file system from ext3 to ext4 on
Jaunty Alpha3. It appears the file system is now screwed (which I don't
care about), as follows:

# umount /t/opt/
# tune2fs -O extents,uninit_bg,dir_index  /dev/vol01/opt
# fsck -f /dev/vol01/opt
fsck 1.41.3 (12-Oct-2008)
e2fsck 1.41.3 (12-Oct-2008)
fsck.ext4: Group descriptors look bad... trying backup blocks...
Group descriptor 81 checksum is invalid.  Fix<y>? yes

Group descriptor 125 checksum is invalid.  Fix<y>? yes

/opt contains a file system with errors, check forced.
Pass 1: Checking inodes, blocks, and sizes
Group 81's inode table at 2654210 conflicts with some other fs block.
Relocate<y>? yes

Error allocating 512 contiguous block(s) in block group 81 for inode
table: Could not allocate block in ext2 filesystem Restarting e2fsck
from the beginning... 
fsck.ext4: Group descriptors look bad... trying backup blocks...

(repeats for ever)

Any hints as to whether this is recoverable before I mkfs.ext4 over it?

-jh