I purchased a Mac pro 1,1 from someone who had previously done an upgrade that allows it to run OS 10.9.5. I'm not sure exactly how they did it, but it has been running 10.9.5 flawlessly for a while now. I have an application for which having 10.6.8 would be handy, and realized that there shouldn't be any hardware restrictions with installing 10.6 on this machine - except for the fact that the super drive has been removed before I purchased it. I have a .dmg for 10.6.7 install, which I restored to a usb stick, and can boot the installer from there. As soon as I select a language, I receive an error message that: "Mac OS X Can't be installed on this computer." I am given an option to restart, or to restore from a time machine backup. If I select the time machine backup, it will allow me to access the command bar at the top and I can get in to Terminal, Disk Utility, etc. (system info and error message screenshots attached) I am unsure if the issue is caused by whatever process made it so that the machine can run 10.9. But anyone who may have any ideas on what to try, your input is greatly appreciated. Here are the things I have attempted: Cleared PRAM/NVRAM Removed all hard drives except for the empty drive to install 10.6 on Removed the EFI partition from the disk using disk utility debug mode after the drive was formatted in 10.9 Still didn't work, so I... Reformatted using disk utility on the 10.6 installation boot disk - partitioned to have a partition less than 1TB (400GB) and made sure that it was GUID Attempted running the installation with only 4GB of RAM in the machine I am at a loss here. I am generally pretty savvy with computers, but some of the real heavy stuff goes over my head. If someone has any directions I can follow, I can easily do that. Is there any reason why Snow Leopard won't install on a Mac Pro 1,1? Is it possible that the EFI partition on the USB stick that was formatted with 10.9 could be an issue? I don't think I can remove that and boot from it, correct?