RFS issues on nandroid restore

Checking MD5 sums...
boot.img: OK
cache.rfs.tar: OK
data.rfs.tar: OK
recovery.img: OK
system.rfs.tar: OK
.android_secure.vfat.tar: OK
Erasing boot before restore...
I:Formatting unknown device.
Restoring boot image...
couldn't find default
Found new backup image: /sdcard/clockworkmod/backup/2011-10-30.07.38.21//system.rfs.tar
Restoring system...
10+0 records in
10+0 records out
40960 bytes (40.0KB) copied, 0.053865 seconds, 742.6KB/s
Illegal instruction
failure while running fat.format
E:format_volume: format_rfs_device failed on /dev/block/stl12
Error while formatting /system!