Re: gptsync: GPT partition of type 'Unknown' found, will not touch this disk.
This is what I did to get Ubuntu working: Nothing.
I figured that I was going to try the stuff in this thread (it all sounds like good advice), so I inserted my Live CD and held down Option as my Mac was booting (this looks for bootable devices by default). My main Mac OS X hard drive shows up, followed by the Live CD, and surprise, so does the Linux partition!!! I booted from it, set up my user account, and I'm typing from Ubuntu right now...
Even doubly better, if I just boot normally without holding down Option, my iMac boots to OS X and bypasses rEFIt (I was planning to do this anyway)...
I know it sounds pretty obvious, but I suggest you give this a quick try before you mess with your partition table... I didn't even try it consciously and it worked. =D
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"The conquest of fear lies in the moment of its acceptance."