EDIT: resolved a two day struggle right after I posted here . By installing 10.6 on the HD, the system no longer throws out any error messages while trying to install LION fresh from a DVD. I'm posting here rather than software forum in hopes that another 2006/07 15" MBP owner can share insight. Just to clarify, it is the model with the ATI X1600 video card. I'm helping a friend who is receiving an error message when trying to install LION on his MBP, and also receives the error if he tries to boot via USB to a known good 10.7 HD. Details: -MBP successfully had 10.7.1 with oem equipment. -he ordered an SSD and began reading about TRIM enabler. Not knowing enough about it he installed and turned it on... with the OEM platter hard drive still installed. -immediately after rebooting, the computer stays at the grey screen with the "prohibited" sign flashing. No luck resetting SMC or PRAM. -decided to perform a clean install, but now LION will not allow him to install. I advised him to find a known good copy of LION burned on a DVD, which he did at a local Apple service center. NO LUCK. -However he is able to install Snow Leopard evidentially. Did he screw something up with the Hard Drive's firmware... OR worse; did he screw up something with the MBP firmware or efi? I am trying here for any insight at all. The fellas at the service center were stumped, and couldn't offer any solutions short of checking it in for full service. EDIT: The error message he receives says "There was a problem installing Mac OS X. Try reinstalling."