iMac Hard Drive Firmware Update 1.0
If you have a mid-2010 iMac, Apple has just released the iMac Hard Drive Firmware Update 1.0 to resolve a problem that might prevent your iMac from booting properly. Although there’s no way to know if you really need this update or not, the inability to boot could make installing this update in the future significantly more difficult, so I’d recommend doing it now, after making sure you have a current backup. (Free, 767 KB)
I just got my iMac back from the Apple store, because I was having some very serious problems booting (except boot camp into Windows 7 which worked great). They had to wipe my Mac OS partition.
so definitely download this update!!