The primary or boot particition is the C: partition. I have never heard the term physical partition. What you can have are logical partitions on a physical driver. For example: One physical hard drive, with three logical partitions, C:, D:, and E:. If you were to go to "My Computer" in Windows, you would see three drive letters even tough you have only one actual hard drive.
The easiest way to add/delete/change partitions without reformatting is to get "Partition Magic." Get that software and it will be very easy.
So to recap: You have a physical drive, and locial partitions on those drives. The main partition can be referred to as the primary or boot partition (C: drive). To create space for a back up you will want to create a new logical partition with the free space on your hard drive.
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