Hard Disk Mirroring (also called as Disk Backup/Disk Cloning) is a process where you can create an exact copy of your existing primary hard disk including partitions to the new hard disk.All data including operating system, drivers, files and programs will be copied sector by sector to your new hard disk. In case if your existing primary hard disk fails you can use the mirrored hard disk to boot your computer. For mirroring a hard disk your need to have one extra hard disk with the same capacity or more. If not make sure that your new hard disk capacity should be more than the total size of all the files and programs (including all partitions) of your existing primary hard disk.
There are many tools available for disk mirroring. Almost all tools will allow you to assign partition size manually to your new hard disk while mirroring, but assigning partitions automatically by the tool is recommended. It is easy for the tool to create size of the partitions based on your existing partitions used space size.