It's usually possible, but awkward, to dual-boot with one OS in EFI mode and the other in BIOS mode. AFAIK, there are only two ways to accomplish such a configuration:
- Use the firmware's built-in boot manager to select which OS to boot each time you boot. This is awkward because most such boot managers are crude, although some are acceptable to some users. OTOH, some EFIs don't even provide the option to do this at all.
- Use rEFInd as a boot manager. If you uncomment the
scanfor
item in its refind.conf
configuration file and ensure that hdbios
is among the options, it will scan for BIOS-mode boot loaders and add any that it finds to the boot menu. This will require you to go from rEFInd to GRUB (or LILO or SYSLINUX or whatever), though, and it's usually easier and more direct to boot a Linux kernel straight from rEFInd. Also, some EFIs lack the necessary support to make this boot method work.
Overall, you're better off booting both OSes in the same mode. The first option can often work, but most people find it awkward. The second option can also usually work, but it's usually no harder to get rEFInd to boot Linux in EFI mode, so there's little point to it.
In theory, it should not be harder to install Debian 7 in EFI mode than in BIOS mode. In practice, though, you'll need to learn some things about EFI-mode booting. My Web page on EFI boot loaders for Linux provides some background information that may be helpful. If you had Windows 8, you might need to disable Secure Boot or work around it in some way, but with Windows 7 that's not likely to be an issue; AFAIK, Windows 7 doesn't include Secure Boot support.
You can't have the PC in both BIOS and UEFI mode at the same time, and switching every time you want to use either OS is I'm sure not acceptable. – Karan – 2013-07-13T03:31:34.183