Some Toshiba (and others) laptops cannot be 'updated' to Windows 8....it is not supported. Did you check with Tosiba before you tried this? Do they have a bios update for your model? Do you have Intel and AMD graphics? If so you can try the Leshcat drivers. You will not be able to install any driver from AMD...Toshiba has customized the graphics to work with their system, and cannot use the 'generic' drivers that AMD offers.
Edit: On the Toshiba page you referenced, there is a link to "Cant find drivers for Win8"...which takes you here. It tells you if your laptop will support Win8 or not.
Intel I7 960 @ 3.87ghz *Intel DX58SO *HIS HD6970 2gb *Corsair TX650M *2x4gb Corsair XMS3 *WD Black 1TB *Windows 7 Home Premium 64bit ***Asus N71Jq laptop *Intel I7 720QM Processor *Mobility Radeon HD5730 1gb *8gb Ram *Windows 7 64bit ** Toshiba P75-A7200 * Intel I7-4700MQ * Windows 8.1 64bit
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08:59 PM by