Hi there! I was asked to help a friend out with upgrading his Mac (mainly because he bought a new rMBP13 with 10.10 running). So we want to keep both (iMac+rMBP) on the same OSX Version. Right now the iMac is running with 10.5.x. So I created an 10.10 USB-Stick - surprisingly I am not allowed to install it on the iMac HDD? While booting with 10.5.x I am also not able to double-klick the installation file on the usb drive - problem with the architecture. I tripple-checked that 10.10 is running on an iMac 8,1 (2GB Ram - this will be upgraded next to 4). I am cloning the HDD now, the completely erase the iMac HDD and try a new clean install ... but somehow I think I will get the same problem? Any suggestions?