In addition to what awokd suggested which I agree with.
Is this the installer boot? or an existing install boot?
If installer boot or after boot from after finished new install, then try re-install with LegacyBIOS/Grub instead of UEFI/EFI, besides UEFI/EFI really isn't any more secure, while LegacyBIOS/Grub is old, UEFI/EFI is crapware, so they even out somehwat, however you may have an easier time with LegacyBIOS/Grub. With little information you get a lot of suggestions though, so there are plenty of other things you can try, for example look here for more things to try out, even if the reason is different, this debugging and fix work-arounds could work for you too:
https://groups.google.com/forum/#!topic/qubes-users/M5laQSahuiE
In order to give more accurate help, we'd need more information. For example questions, such as like those I asked the other guy in the link.