disk block failing on root disk on sun blade 1000

[fddi]

Hello,
ysterday I did a

df -sh /

on my sun blade 1000 and the machine crashed.

when I could read the console this morning, the boot process was stuck 
telling root partition was not clean

I ran fsck manually and the problem is that fsck finds a unreadable 
block on disk.

so fsck cannot fix this.

I can only mount the root filesystem forcing it.

that is bad because the root FS is unusable and the machine can't boot.

how can I fix it ?

I was thinking to force mounting and try to backup filesystem.

so my question is how can I backup the rootfs and restore it with its 
boot block when I replace the root disk partition with a brand new disk ?

thank you

Rick