Prior to making any changes to your computer’s drive you need to establish a baseline backup of the entire drive in case you need to restore Windows 8.1 to its current state (containing all your data files and applications) for whatever reason after installing Windows 10 or you are no longer able to boot to Windows.
The screenshot below shows a typical 500GB drive with 3 partitions (UEFI initialized as GPT) when using the Disk Management utility, please keep in mind that your PC’s partition arrangement may be different.
The very first partition is a small 300MB Recovery partition which is normally hidden when using File Explorer, the second partition is the “System” or “EUFI System” partition (also hidden in File Explorer) which Windows relies on to boot. If this partition were to become corrupted you need a way to restore it, which would be from your image backup. The third and final partition is the Windows 8.1 (C:) partition.
Note: if your disk drive contain a larger “Recovery” partition provided by the computer manufacture, you can use it as a means of restoring Windows 8.1 back to a “fresh out of the box, when you first turned your computer for the first time” state, note that using the built in recovery function will result in all your data being lost and any applications you have installed will need to be reinstalled. The image backups you will create over time as you evaluate Windows 10, eliminates the need to use the recovery partition to reinstall Windows 8.1, your applications and data files. It’s your safety net when your evaluation of Windows 10 has been completed or the preview has expired (April 15, 2015).
My personal style of where and how to store the Image Backup file includes the use of an external USB drive and a destination folder name (created prior to launching the image backup software). The folder name should have some meaning (Example: Windows 8 Baseline Backup). Inside this folder I also create a small Notepad Readme.txt file that includes notes about why and when the backup was created.
The time to complete the backup is dependent on the image compression level, amount of data / used drive space your Windows 8.1 partition contains and to some extent the size of your recovery partition. I choose Medium compression as it provides a good balance between the size of the image backup file on the USB drive and the time to complete the backup. Higher compression levels usually do not save all that much space and can add significant time to the backup completion time.
After your baseline backup has complete, the next step is to shrink the existing Windows 8.1 partition to make room for an additional partition to be used when installing Windows 10.