Installing Debian: Force UEFI, what does it affect?
You're in luck because as part of my own adventures, I had a similar question to you. Forging ahead, I just did the following:
- took a machine with Windows 7 installed in legacy BIOS compatibility mode on HDD0 and an unused data drive on HDD1 - leaving these drives connected
- toggled its firmware to UEFI mode
- installed Debian 8/Jessie in UEFI mode on an mSATA SSD
- chose "Force UEFI", again assuming the option A outlined in my comment above
- hoped for the best
Everything was fine. To boot Debian, I leave my BIOS in UEFI mode. To boot Windows 7, I toggle back into legacy mode.
Thus, I have to assume my option A above was correct: "Force UEFI" seems like a poor way of wording the proposition 'Some firmwares might get horribly confused if you install a UEFI/GPT bootloader on one disk, while having BIOS/MBR bootloaders elsewhere on the system. Would you like to give up, just in case?'
Having said that, for anyone thinking my setup is a tad confusing, you're right - and this experience has shown my firmware might well be one of those poorly programmed ones - to say the least. But I don't believe that specific anecdote is relevant here, so I'll stop myself. The key thing is that it works - for now!