First, it's conceivable you've got Secure Boot enabled. To the best of my knowledge, only Ubuntu 12.10 really supports Secure Boot yet, although support is in the pipeline for several other distributions. If this is correct, disabling Secure Boot in the firmware might fix the problem. This explanation is unlikely, though, unless there was a typo in your description, since Secure Boot has become an issue with Windows 8
. To the best of my knowledge, no Windows 7 systems shipped with Secure Boot active, and AFAIK the standard Windows 7 boot loader doesn't support Secure Boot. Thus, it might be worth checking your firmware, but don't spend too much time on this. (Indeed, until Windows 8's release, few EFI implementations even included Secure Boot options.)
Second, it's possible to install Mint (or any Linux) in BIOS mode and then switch it over to use EFI-mode booting. I've got an LMDE installation that I set up in this way. The procedure is, in brief:
- Launch the installer in BIOS mode.
- Install normally, making sure you're using a GUID Partition Table (GPT) for your partitions, not the older Master Boot Record (MBR) partition table. If you're installing to a disk that already holds Windows, this should be the way it's set up already.
- Use the firmware's boot options tool or something like Super GRUB 2 Disk to boot into Mint.
- Install an EFI boot loader for Linux. This is where it gets tricky, since you can't do a complete installation from BIOS mode. You can, though, install the files and then finish the job later. Also, installing the grub-efi package will remove the grub-pc package, which you may need to boot into BIOS mode, so you might not want to do this just yet. Overall, you might want to try ELILO or Fedora's patched GRUB Legacy, at least as a stopgap measure. See my Web page on EFI boot loaders for Linux for more complete installation instructions.
- Reboot and use whatever means is available to boot the just-installed EFI boot loader. This is another tricky step to describe because there are so many possibilities and many of them are system-specific. The method that's most likely to work on most systems is to prepare a bootable external disk (CD-R or USB flash drive) with my rEFInd boot manager. (There's a download link to a bootable CD-R image file; or the install.sh script enables you to create a bootable USB flash drive.) With any luck, Mint will boot in EFI mode.
- Either use efibootmgr to add the boot loader you installed earlier to the NVRAM settings or replace grub-pc with grub-efi. This will make the system boot Linux in EFI mode. Note that if you use ELILO, it can't chainload to Windows, so you'd need to add rEFInd or gummiboot to the mix, or perhaps rely on your firmware's built-in boot manager (but most of them are very poor).
As you can see, steps #4 and #5 are a bit sticky for the uninitiated, but it is do-able. It will be easier in the future -- and in fact it already is
easier for some distributions -- because the 3.3.0 kernel includes its own boot loader. That, in combination with drivers and features provided with rEFInd (especially in version 0.6.0 and later) enables changing steps #4 and #5 into something much simpler -- you can install rEFInd and use it to boot the Linux kernel directly. If I'm not mistaken, though, LMDE still uses a pre-3.3.0 kernel by default, and this approach requires a 3.3.0 kernel.
Ubuntu also has a tool called "boot repair" that can be used to do the work of steps #4 and #5; however, I don't know if it would work with LMDE. If it doesn't, it could make matters worse, so I'm reluctant to recommend using it.