OS X V10.4 :: Hdiutil Create Srcdevice Fails To Create Image

I want to create a disk image of a Mac drive (.dmg file).I figured that it might not be a good idea to try and image a live disk, so I unmounted it (the user dragged the disk icon to the eject icon), and I confirmed it was unmounted by looking at /Volumes. To be clear here, this isn't the boot volume, that is /dev/disk0./dev/disk1 is an internal drive - as it happens it's the original drive for this OSX 10.4 G5, AND /Volumes/HITACHI is the mount point for an external USB drive.I have tried this twice.On both occassions, things seemed to go well, but after a few minutes (about 10 - 20 minutes), I observed that the target file on /Volumes/HITACHI stopped growing, then a bunch of unpleasant stuff started to happen: 

* I did lsof /Volumes/HITACHI/MacHD_20120201.dmg and found the PID of the diskimage binary, ps waux showed it was doing nothing (CPU% was 0.0). This indicates a stuck process to me.

* The hdutil command stopped producing '.'

* I could not cancel the hduitl command

* I could not rm the MacHD_20120201.dmg file

* I could not ls the /Volumes/HITACHI directory

* unlink also hung 

A little further into this, the Mac became unresponsive. I observed that the disk was now mounted again (grrr!). As a work around I switched the USB drive off and on, and Mac came good - all my terminals started to respond, the GUI responded for the user.My workaround has been to use CCC to make a clone of the disk - but I don't like this because I think .dmg files are much more convenient (for a start, now that I've used CCC I can't use the USB drive for anything else).

* Is my methodology wrong? Do I really need to umount the /dev/disk1

* Can you disable finder from automounting an internal hard drive (I am particularly interested in this) 

Info:
Mac Pro, Mac OS X (10.4.11)