Open disk image created with dd

Hi!

To make a full copy of a friends HFS+ partition, I used

*dd if=/dev/disk1s5 of=/Volumes/JH Data/JHMess1.dmg conv=noerror,sync bs=1m*

Now I am trying to mount the new disk image, but hdiutil fails with

*# hdiutil attach /Volumes/JH Data/JHMess1.dmg*
The disk image you are opening may be damaged and could damage your system.
Are you sure you want to open this disk image? (Y/N) y
hdiutil: attach failed - no mountable file systems

The original /dev/disk1s5 mounts without any issues. Did I miss to copy something important?