OK, so I have this cheap and cheerful laptop from Acer and I want to use Debian on it. The problem is that I also want to keep the Windows install for both warranty reasons (for ease even if not legally required) and because the Windows install has its uses at the moment.
I have googled this but the majority of what I see about dual booting Debian with Windows 8 seem to be videos which make assumptions and must be followed to the letter.
So, please, does anyone have any advice on how I can proceed without nuking the Windows install? It has a 500GB (spinning rust, unfortunately) HDD so there is room but I fear that messing with the partitions will incur the wrath of secureboot and I'll lose Windows. So, for example, can I take a backup to a USB stick in such a way that I will be able to restore my system should the inevitable happen?
Thanks in advance for any ideas and advice.

I wasn't sure whether to post this under the laptop forum or the Debian forum but I think it is best here as I would like to run Debian not whatever is easiest.

My Asus X551MA has similar specs. The first thing you should do is to go in to BIOS and turned off quick boot and secure boot since they make it where only Windows can boot. It will still be UEFI, and Windows should still boot. Now the tricky part - getting a UEFI Debian installer on a USB stick. (By the way, what version of Debian are you planning on using?)

Many thanks for the reply.
While it is obviously cheap I to like the machine so far (apart from the capacitive touch pad but that's another story) though as somebody who used to work for a VAR and take Acer warranty calls fairly regularly I do worry about longevity.
I found my way into the BIOS and managed to boot a (Wheezy, I think it was) USB stick so I have that covered, thankfully. At one point before that I changed a BIOS setting which looked like it turned off secure boot but the machine still booted into Windows so I'm not sure what it really does.
The aim is to run Sid as I tend to do when I can.

Now I'm really stumped.
I switched to legacy boot and booted successfully booted the Debian 7.6 live DVD image from a 2GB USB2 stick, then inserted a 32GB USB3 stick (for some reason the machine only sees the 2GB stick to boot from when the 32GB one is absent) and ran the installer selecting the 32GB stick as the target. All went well until the GRUB installation when the installer insisted on trying to "install GRUB to master boot record" but by that it meant /dev/sda rather than either the drive it booted from or the drive I was installing to. Is this a "feature" of the Debian installer that it isn't able to install a boot loader to a partition of choice but just defaults to something at random?

You're in legacy mode, which means it wants to install an MBR grub, not a gpt grub. MBR grub will only want to be installed into the MBR of the disk unless you tell it you are using another bootloader.

Well, yes, I am in legacy mode but then why can't the installer install GRUB to the MBR of the disk it managed to install the whole OS to? My secondary question was why can't I choose which disk to install the GRUB loader to?
Do I turn off Secure Boot but leave EFI on then boot fro ma Debian installer instead?
I even tried booting into the live USB then chrooting into the install and trying install-grub from there but whilst it didn't complain it also didn't work on reboot.

It's going to be easier to chroot with a disk that will boot to efi like ubuntu or debian testing.
Did you create an efi-partition?
There should be a PQSERVICE partition, you can either make an image of that, or the whole system. But in general all you need is the PQservice to restore to factory condition.

He's in legacy mode, no more EFI booting.

As above, I'm doing this in legacy mode. Does the installer give an option to create an EFI partition? I don't mind losing a GB or two for an EFI partition though I'm not even sure the laptop will boot from the install USB in EFI mode at all?
It would be quicker, for now at least, though if the installer let one choose where to put GRUB. Is this something that was phased out in the name of simplification? Should I be using an alternative install CD or DVD image?

You can always try this as I stated in first post
https://www.youtube.com/watch?v=DLlOd-a2wG0

I do appreciate the help but that is the video I was referring to in my original post. The video assumes a partition layout that differs from my own and, since it's installing GRUB to the machine's hard drive, carries the real risk of preventing Windows booting at which point my machine may well no longer have a warranty. This is why I was asking whether there are any text instructions along similar lines so that I can work out where my situation differs in advance and try to plan things as I find it hard to do so from a video.

The reason why you experienced the problem here is using the live-install. You would be better off using the standard dvd,iso. I used the live-cd a few days ago & it installed everything without prompting after partitioning.
You have 3 choices chroot, reinstall with standard wheezy iso or forget it.
The chroot method does work? what is the difference in your partitioning scheme?
If you choose to install with standard iso, use dd instead of unetbootin so you can get the advanced options. Although I believe if you use standard install you still can choose where to load bootloader.

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Thank you, I will grab the standard installer and try that -- I used teh live installer as it's the most up to date .iso I happened to have on my desktop.

I'm not sure which one I found, but I did find a UEFI installer of Debian that worked great (and before that I found 3 that didn't work at all). I've used it to successfully install my laptop #3 in dual boot with Windows 8 (since that firmware actually doesn't have a legacy mode at all).

There were some debian iso's that were listed as DI1-3 before jessie but now I think the 64-bit testing iso has it by default. Also if you're using testing you don't need unetbootin to create usb, just right-click on iso & select Disk Image Burner which works great with usb, because if the iso has efi it will create an efi partition also. The only thing is I'm not sure if you have to change bootflag to the efi partition .