Comment 18 for bug 1015211

As a practical matter there really should be an override for the installer. I suspect Win7 will continue to exist for a number of years and with that there will continue to be UEFI systems that are configured to boot /dev/sda in bios mode but which will by default be configured to boot any other media it sees in EFI mode. Its really confusing to explain to someone that they have to go into their EFI firmware and fiddle with boot options to get the CD/USB keychain booting in bios mode.

My suggestion would be:
a) One the partitioning screen a button that says "My other OS is not shown" which then scans for an mbr table and shows the non-bootable Windows partition.
b) A quick bit of text that says "This operating system seems to be configured in a way that will not boot. If you know this is incorrect you may override the installation"
c) If the user clicks "yes" continue the install but assume that the system is a BIOS partition ignoring all evidence to the contrary.