Windows to Go, Goes Into Black Screen After 2 Minutes with Late 2009 iMac (Mobility Radeon HD 4850, GPU)

Hi. If anyone has experience in successful using Windows to Go, using a late 2009 iMac (with 4850, GPU), I'd be very helpful to know how you went about it.

My Windows to Go, whenever I install it, using Boot Camp, Windows Enterprise 8 64-bit, using the Windows to Go app to a USB external hardisk (more than 140GB), it would work until after around 2 minutes, by that time, it would go into a black screen but is still somehow alive coz' the iPhone I would connect to it, still charges and sometimes, it displays the "Trust" this computer prompt. I've checked, it's not the energy saver.

If it's a GPU driver issue, could you point me to the exact download location, perfect for this iMac, for Windows to Go (Windows to Go is a great way to just have one partition on the iMac).

Thank you in advance. Happy new year, belated (new year sharing: Rev. 21:4) and have a great upcoming sunday mass/great week.

God bless.