Windows 10 Technical Preview is great! I had it installed on a virtual computer, via VirtualBox, but as the updates kept coming, and I kept using it more often, I decided to dual boot it with Windows 8.1. It wasn't as simple as I thought it would be, but here in this post, I will go through step by step how to do it, and what issues you may encounter.
The first step you need to do is to make a partition for Windows 10 Technical Preview. To do this, you have to open File Explorer, and click on 'MANAGE', which should be on the top right corner. Just look around, it's pretty small. Once you click on that, a program called Computer Management should come up. Go to Storage, and click on 'Disk Management'. Under Disk 0 (it might be different depending on your computer), you have to right click on the C, or where your current OS is installed. You then have to click on 'Shrink Volume' Wait a bit, and choose how much MB you would like to devote to Windows 10 Technical Preview. I put 50,00MBm or 50GB, but the amount of space is up to you. Don't worry, you can delete this partition later if you want. Then, click on SHRINK. Once that's done, you should see something on Disk 0 that is black, and says Unallocated. You want to right click that, and click 'New Simple Volume' Keep clicking next. I will ask you which drive path you would like, but any letter would be fine. Finally at the end, just format the partition, and you are done with setting up the partition for Windows 10.
Next step is easy as well. Click here to get redirected to Microsoft Windows 10 download. I made it open to a new tab, so you won't lose this page. You may have to sign in, but scroll down to 'Download Links", choose your language and country, and download the ISO file. It might take some time, but make sure you know where you save it, and give it patience and time. If you are using a 64-bit computer, choose x64, if not choose x86.
This is probably the harder parts. It's more tricky than hard, and it's where I got tricked up. If you use Window's USB Download tool for ISO, it won't work. In short, when you start setup, it will tell you that it is unable to run with GPT partition. You have to click here, it will open in a new tab. You scroll down to 'Download', and lick on Rufus 2.1. It may be a different version when you download it, but that is what you download. You have to first insert the ISO file, look at the image below to see where to click.
Now, you have to plug in a USB thumb drive that is 8GB or larger. I used a 4GB flash drive since it was the only one I have that was empty, but it might run out of space, depending on the version you download. Be warned that this will format your whole flash drive. You choose your ISO file, the one you downloaded earlier. Then, you have to change the Partition Scheme and target system type to GPT partition, UEFI. This is important, because if you don't change this scheme, you will waste your time trying to install a GPT on an MBR file. You just wait and wait more, until it is done.
This is almost the last step! You have to plug that USB into your computer. Then, you have to turn off your computer, and press F12 continuously. Some computer will have a button to press, and I know that Lenovo laptops will have a 'OneKey' button, or also called a 'Novo' button. This can be located on the sides of your computer, or right next to the power button. If you can get to the Boot menu and change to boot to the USB flash drive, then great. Move on to the next step. If not, continue reading. You will have to use your arrow buttons, and hit enter at 'Enter Setup', or simply 'setup'. You have to go to the boot menu, or the start. Something along those lines. You can contact your PC maker for more help, since every computer is different, but with most Lenovo computers, you have to first disable CSM, so you can get to your boot menu settings. In the images below, you will see that with CSM disabled, you cannot change Boot menu settings. And with Boot priority, you want UEFI First. Your hard drive is most likely defaulted to this setting, but you need this to be first, not legacy. You then go over to the exit tab, and save settings. Then restart, pressing F12 again, or the Novo button. Again, ask your manufacturer if you are not sure. Now, you should be able to choose your flash drive as the boot device.
Keep clicking continue, next, or something along those lines. Accept the terms and conditions, and finally, choose the partition that you created. Remember how much GB the partition was, and click on it. That is the one where you will install Windows 10 Technical Preview.
Now, there are some things to know. When you turn on your computer again, you will have have two options to choose from: Windows Technical Preview and Windows 8.1. This is called dual-boot. Once you are done with this preview, or the real release came out, you can easily delete this partition, in which I will put a new blog post soon. There are some things to do before you can enjoy it. First, make sure that you install ALL updates. You should also change the update mode to fast so you get all of the latest builds. You simply search 'Update', open Update & Recovery, and click Advanced Options. Scroll down to 'Choose how previous builds are installed', and change that to Fast. Not mandatory, but I would suggest it. I mean, who wouldn't want to try Project Spartan and Cortana?
DISCLAIMER: We are not responsible if you do any harm to your computer, even though this should most likely do absolutely nothing bad to your computer. Make sure you follow each step carefully.