iMac in Target Disk mode won't mount on Mac Pro via Firewire

[abcd-abcd]

My iMac (intel Core 2 Duo) won't start up - it has the question mark in a folder icon.
I can't find the Apple Snow Leopard disk, so I've started it in Target Disk mode, connected it to my Mac Pro, but it doesn't mount on the Mac Pro even though it appears in System Profiler on the Mac Pro.
Both running 10.6.8
Can I force it to mount? I have tried front & back firewire ports and restarted several times to no avail.
I'm hoping to repair the iMac disk using Disk Utility or Diskwarrior on the Mac Pro, but need it to mount first.
Any ideas? Thanks.

 

[simsaladimbamba]

Disk Utility does not need to mount (making it visible for Finder) the HDD to repair it.
But the question mark points to an OS not found, maybe due to a corrupt OS or partition map or even HDD.
If you can see the HDD in Disk Utility, can you repair it?

 

[abcd-abcd]

Thanks very much indeed for your reply. Eventually I managed to get it to mount and now Disk Utility is trying to repair it.

 

[simsaladimbamba]

Thanks very much indeed for your reply. Eventually I managed to get it to mount and now Disk Utility is trying to repair it.

What do you mean with "mount"?