unable to set superblock flags

An ext3 filesystem would not mount today :(
Running e2fsck (under MacOS 10.4/ppc) only gave:

$ e2fsck -fv /dev/rdisk0s4
fsck.ext3: unable to set superblock flags on /dev/rdisk0s4
Then I remembered to try with a dfferent superblock, and it worked perfectly:
$ mkfs.ext3 -n /dev/disk0s4 
Superblock backups stored on blocks: 
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632
Note: Don't forget the -n option, otherwise mkfs.ext3 will really format the partition!
$ e2fsck -b 32768 -fv /dev/rdisk0s4 
$ mount_ext2 /dev/disk0s4 /Volumes/disk0s4/
Now mounting succeeds again. Yeah, ext3 ;-)