I have a intel imac from 2010. About a week ago the imac stopped booting osx but continued to let me use windows on a separate partition of the same hard drive. I have erased, reformatted, verified and repaired the hard drive using disk utility and reinstalled osx (10.6) the smart status is also verified.

The mac will still not boot. However if i put the imac into firewire mode my macbook will boot using the imac's hard drive. Ruling out a problem with the hard drive itself.

Also putting the macbook into firewire mode and booting the imac from the macbook's hard drive also works! So i really dont know what the problem can be?

What happens and how far does it get when you try to boot into OS X? You can hold down the option key on your keyboard to get a list of the bootable partitions. If OS X doesn't show up there, that partition is corrupt enough to no longer be detected as a bootable partition, so you'd need to reinstall the OS.