Try to find out what filesystem there is on the partition.
<code>
dd if=/dev/hda7 of=yy bs=100k count=1
</code>
You need to be root for that. Then type
<code>
file yy
</code>
In my case this yields "yy: Linux rev 1.0 ext3 filesystem data (needs journal recovery)". The error part comes, of course, from the incomplete copying.