hdiutil giving me no love

I've used the hdiutil several times before but this time it has me abusing keyboards.

I'm trying to attach a damaged disk image to the system but not mount it in the Finder so that I can use fsck_hfs on it.

This is the command I use(d)
hdiutil attach -nomount -ignorebadchecksums -noverify -noautofsck -readwrite [diskimage path here]

The error is recieve is
hdiutil: attach failed - Operation not allowed

This seems to happen on any and all disk images - even working ones!

Any ideas on how to solve this?
PS: It does not present any errors if the -readwrite parameter is removed. But I need the -readwrite option in order to repair it.

Thanks