I encountered the same problem last week. I use MDT to build our PCs and it injects the relevant drivers before the actual build starts and is done over the network and the problem started when we added the driver packs for the 790.
It appears that the chipset drivers for the 790 are compatable with the 620 and because it is a newer version decides to use these.
The problem seems to be that because the 790 doesn't have PATA connections just SATA it doesn't require any drivers for them so when a 620 reboots after installing the chipset it fails when trying to connect the CDROM which is connected to the PATA connection.
because MDT is done over the network i was able to just disable the PATA in the Bios and it then built without any problems.
i then installed the chipset for the 620 to install the PATA driver, rebooted to complete the install rebooted again and reenabled in the bios and all then worked fine.
currently trying to find the PATA driver to inject at the start of the MDT process so don't have to mess about with disableing and installing drivers again.
i know this won't help initially for people building PCs using CD but if you can find the PATA driver and then include in your install i believe it will solve your problem.
Hope this helps