I'd like to try Windows 10 Technical Preview before it is released for purchase. I am curious and have never used a windows based machine before. Question: Is it possible to install and run Windows 10 Technical Preview under BootCamp
I will offer the following link for to the drivers needed to install windows for your machine.
You will need to download the zip file found at Boot Camp Support Software 5.1.5621
So it can take a bit of work but it is possible, you don't need to use bootcamp. I followed this guide and have had Windows 7, Windows 8 and Windows 8.1 running from a USB3 SSD in the past. I am going to attempt to use this guide to install Windows 10 in the coming days.
Pasted directly from link below......
What you'll need
An external hard drive with a USB3 and/or Thunderbolt connector. This drive will be formatted so ensure you saved your files before going further. You can use either an SSD drive or a classic hard drive. A Windows 7 or 8 install DVD or ISO (check whether to install 32 or 64 bits versions based on your Bootcamp drivers) and the corresponding Windows serial number.
One of the following:
Mac OS X with a Windows 7 or 8 Virtual Machine (use VMWare Fusion or Parallels Desktop for example. Note: VMWare Fusion seems to have some issues with Thunderbolt and USB3. Plug your drive to a USB2 enclosure or hub to work around this -it worked for me-, or use another VM software) → Read the important note below A PC running Windows 7 or 8 → Read the important note below Windows AIK (free) running on your Virtual Machine or on your PC, or just the imagex.exe file (the rest of the Windows AIK package is not needed) Download imagex.exe Download Windows AIK (this download and installation is not required if you have already downloaded imagex.exe) Bootcamp drivers for your Mac. You can get these either by running bootcamp from your Mac (Applications > Utilities > Bootcamp) or, if like me you have a 3TB drive and can't run bootcamp at all, use the direct download links here. A USB stick to store your bootcamp drivers IMPORTANT: If your Mac has a 64 bits processor, your Windows Virtual Machine on OSX, your Windows installation on your PC and your Windows DVD/ISO must also be in 64 bits! Step by Step guide
Step 1: Get the install.wim file
If you have a Windows ISO file: Mount the ISO If you're on OS X: double click on the ISO file If you're on on Windows 7: Use a software like Virtual Clone Drive (free) If you're on Windows 8: double click on the ISO file Open the mounted drive, then go to the "sources" folder and locate the "install.wim" file. Save this file to C:\wim\ on your Windows installation or virtual machine. If you have a Windows DVD: open the "sources" folder on the DVD and locate the "install.wim" file. Save this file to C:\wim\ on your Windows installation or virtual machine. IMPORTANT: If instead of a "install.wim" file, you have "install.esd", you can not continue this step by step guide. And an ESD file can not be converted into a WIM file. So you must get a version of the Windows installation DVD/ISO that has an install.wim file. Step 2: Clean, partition and format your external hard drive
On your Windows installation or virtual machine, plug in your external hard drive (can be plugged using USB2, USB3 or Thunderbolt at this stage) Open the command prompt in administrator mode (cmd.exe). To run it in administrator mode, right click on cmd.exe > Run as admin. Type the following and hit enter to open the disk partitioner utility: diskpart Type the following and hit enter to list your drives: list disk This will display a list of disks mounted on your computer or virtual machine. Make sure your drive is listed here before you continue. Identify the disk ID of your external hard drive. Replace # by your real external disk ID in the command below: select disk # Clean all partitions by typing the following (warning: this will erase all data from your external drive!): clean Type the following to use MBR as partitioning format: convert mbr Create the boot partition by typing the following followed by the enter key: create partition primary size=350 This will create a 350MB partition on your external drive Format the partition in FAT32 by typing the following: format fs=fat32 quick Set this partition to active by typing: active Assign a letter to mount this partition. We will use letter B in our example. If B is already used on your PC, replace B by any other available letter: assign letter=b Windows will detect a new drive and probably display a pop-up. Ignore that. Create the Windows installation partition using all the remaining space available on the external drive by typing the following: create partition primary Format the new partition in NTFS: format fs=ntfs quick Assign a letter to mount this partition. We will use letter O in our example. If O is already used on your PC, replace O by any other available letter: assign letter=o Windows will detect a new drive and probably display a pop-up. Ignore that. Exit the disk partitioner utility by typing: exit Step 3: Deploy the Windows installation image
Still using the command prompt in admin mode (you didn't close it, did you? ;) ), locate the imagex.exe file mentioned in the "What you'll need" section and access its folder. In our example, we have put this file in C:\imagex\imagex.exe Your Windows installation ISO has probably several installation options in it. So in order to identify the right one, type in the following: imagex.exe /info C:\wim\install.wim Now identify the INDEX number of the installation that you wish to deploy. You will use it in the next step. Type the following and hit enter. Replace 1 with the INDEX you identified in the previous step. Also remember to replace o: with the letter you have chosen in the previous step: imagex.exe /apply C:\wim\install.wim 1 o: This will take some time. The Windows installation image is being deployed to your external drive Once done, type the following to create the boot section (remember to replace o: and b: with the letters you've chosen in the previous step): o:\windows\system32\bcdboot o:\windows /f ALL /s b: If you get an error message saying that you can't run this program on your PC, then most probably you are running on a 32 bits installation of windows and you're trying to deploy a 64 bits install. This means you did not read the important notes in the beginning of this guide ;) If you get an error message on the options that can be used with the BCDBOOT command, then it's because you're installing Windows 7, and the /f option is not supported. If that is the case, remove /f ALL from the command and retry. Step 4: Boot from your external drive and install Windows
Plug in your external drive: If you've done all the previous steps from a Windows PC, unplug your external drive from your PC and plug it to your Mac, either on a USB3 or a Thunderbolt port. If you've done all the previous steps from your Mac using a Virtual Machine, ensure the external drive is plugged in to a USB3 or Thunderbolt port. Using USB2 should also work but you'll get very poor performance so I don't recommend doing that.
Reboot your Mac and once the bootup sound is over, immediately press the ALT (option) key and release it only when the boot drives selection screen appears. If you did not get the boot drives selection screen, reboot and try again. The timing to press the ALT (option) key is quite short. It must not be too early or too late. On the boot selection screen, choose "Windows" using the arrow keys on your keyboard, then press enter. The Windows installation starts. Follow the on-screen instructions as normal. The installation program will restart your computer one or 2 times. Don't forget to press ALT (option) right after the bootup sound, and boot on Windows again each time to continue the installation. Step 5: Install bootcamp drivers
Once the Windows installation is complete, plug in the USB stick where you stored the bootcamp drivers (see "what you'll need" section), open it and right click on "setup.exe" and select "Run as admin". Follow the on-screen instructions. If you have an error saying that you can't run this program on this PC, obviously you have installed a 32 bits version of Windows and the bootcamp drivers for your Mac are made for a 64 bits version. You have to restart the whole guide and make sure to get a 64 bits version of Windows this time! Once the bootcamp drivers are all installed, reboot and press ALT (option) after the bootup sound to boot on Windows again. And Voilà, you have Windows installed on your USB3/Thunderbolt drive running on your Mac. Now each time you want to boot on Windows, press and hold the ALT (option) key after the startup sound and select "Windows", then press Enter. You can now play your favortie PC games :)