I've got a laptop here that came with a Windows 8 installation. What I did: Removed the harddisk, put in an SSD and now I've tried to, unsuccesfully, install Debian. I've used all the default options of the Graphical Installer and chose not to force UEFI. However, it doesn't boot but hangs in PXE trying to start via IPV4.Now, I'm revisiting the installer and wondering: If I choose force UEFI, does that only affect the installation on the SSD? Can I swap back the harddisk and will my Windows 8 installation automagically work, or will force UEFI do other things I don't know about, that will make it impossible to swap back my harddisk at a later time? 
Installing Debian: Force UEFI, what does it affect?
debian;boot;uefi;debian installer
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 connectedtoggled its firmware to UEFI modeinstalled Debian 8/Jessie in UEFI mode on an mSATA SSDchose Force UEFI, again assuming the option A outlined in my comment abovehoped for the bestEverything 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!