Had same problem; seemed to be due to creation of a (uefi) usb startup disk using unetbootin on a mac. I solved it by selecting "try ubuntu" (not install), and then from using firefox to download the amd64 12.04 iso and using ubuntu startup disk creator to create a second usb startup/install flash disk. Reboot with this new second flash disk, hit escape, select the non-uefi flash drive to boot with.. and install ubuntu (I selected the erase all previous partitions option). Presto!
Had same problem; seemed to be due to creation of a (uefi) usb startup disk using unetbootin on a mac. I solved it by selecting "try ubuntu" (not install), and then from using firefox to download the amd64 12.04 iso and using ubuntu startup disk creator to create a second usb startup/install flash disk. Reboot with this new second flash disk, hit escape, select the non-uefi flash drive to boot with.. and install ubuntu (I selected the erase all previous partitions option). Presto!