Since this post is a snapshot in time. I recommend that you download a copy of the book which is updated frequently to improve and expand the content.
Backup the Raspberry Pi SD Card
Once we have installed our base of software it is a good idea to copy an image of the SD card onto the Windows machine so that we can recreate (clone) the installation back to the current state.
We will use the Open Source utility Win32DiskImager again. Use the SD card reader capable of accepting your micro SD card and note the drive letter of the SD card.
Start the Win32 Disk Imager program.
|Win32 Disk Imager|
Select the correct drive letter for your SD card (make sure it’s the right one) and enter a new name and location for our new Raspbian disk image. Then select ‘Read’ and the disk imager will read the image from the SD card and create a new disk image on the windows machine at the location you have specified.
Once the process is finished, exit the disk imager and eject the card from the computer and you’re done.
The post above (and heaps of other stuff) is in the book 'Raspberry Pi: Measure, Record, Explore' that can be downloaded for free (or donate if you really want to :-)).