If you are doing a fresh install by formatting the primary boot partition ie locally the c:\ drive then yes
or if you r going for an update, then it will change the system files to its next version without disturbing the files and settings of ur system...
You can completely clear your external hard drive by formatting it.
No it does not. As long as the motherboard has the proper connectors for the hard drive you are installing, IDE or SATA, you will not have to replace it. If the connectors are different, then return the hard drive and get one with the proper connection type.
operating system
the hard drive should be set to master...
I would suggest installing windows on one hard drive, then installing ubuntu on the same hard drive. Use the other hard drive for the backups (partition it).
Installing the program to your hard drive
without a hard drive your PC wouldn't work at all. so you must have a hard drive, whether its internal or external...
You normaly only need a srew driver
yes all hard drives must be reformatted before installing an operating system.
yes you do.
Defragging the hard drive
configure the bios settings to auto detect the master hard drive