You can get the best answer from Lenovo, since they know the most about that particular computer. However I've seen a good number of reports here from folks who've tried doing what you did and run into similar problems. The problems are related to the BIOS and the way the hard drives are set up. If you put Windows 8 back on that computer, install Start8 or Classic Start, and it'll take a lot of the pain of Windows 8 away. They give you the old Start menu back with the look and feel of most Windows 7 features.
I just bought a Lenovo z500 and formatted it to get windows 7. I downloaded all the drivers from lenovos driver site (The right drivers!) I copied them with a usb-pen and tried to install them on my z500, the bluetooth driver worked, the rest of them gave me different kinds of error messages all saying something like "Driver not compatible for this system" or stuff like that. I have downloaded the drivers several times and tried with different copies of windows 7. Nothing worked.
Is it impossible to get windows 7 for my z500 - meaning am I forced to use windows 8?
And if so does my z500 remember my product key, because I can't seem to find on the back of the computer, like I'm used to.
Tobias Jensen, Denmark.