DVD's I'm not sure but I copy my CD's to my hard drive using Windows Media Player or any other software that converts a song from CD format to MP3 or WMA formats. A MP3 or WMA file is about a tenth the size of the original. I have over 4 thousand songs on my hard drive and it only takes up about 15 GB of space. I don't think you can back up DVD's without special pirating software.
I am guessing you mean "Do backups stay in the PC after Format?" The answer is No, if you store the backups of the files on the same hard drive you are formatting. Its not technically a backup then anyway. To make a proper backup, you need to copy your files to a USB key, burn them to a CD or DVD, or copy them to another computer or hard drive.
External hard drives do nothing on their own. Their purpose is for you to put data onto them. You could very well copy DVDs and put them on the hard drive, but they aren't going to do that by themselves.
You should perform regular backups of a hard drive at least weekly.
Hard copy is like a audio CDs and DVDs
Daily backups are ideal.
Same thing as in Windows. You need a DVD copy software to help you, I suggest you use Longo Mac DVD Copy from http://www.longodvd.com , it can automatically decrypter copy protections of DVDs and copy DVDs to hard drive or burn DVDs to blank discs easily.
If it's DVDs you want to back up to your hard drive, use DVD Shrink 3.2 or DVD Decrypter (Freeware, can be found with a search engine).
weekly
Yes.
external hard drives
To rip Blu-ray DVDs to a hard drive, you'll need software capable of handling Blu-ray formats, such as MakeMKV or HandBrake. First, install the software and insert the Blu-ray disc into your computer's drive. Open the ripping software, select the Blu-ray source, and choose the output format and destination on your hard drive. Finally, initiate the ripping process, and the software will copy the contents to your hard drive.
No. No it won't.