you backup before formatting your hard drive because when you format it gets rid of EVERYTHINg, so essentially you COULD backup after you format, but then you would have nothing to format, so backup before, then format, then put all the backed up data back
you copy all the original files to a another hard disk or flash drive before formatting the disk with the original files
Of course, copy all your programs and files you need from the old hard drive and put it on your new hard drive BEFORE formatting the old hard drive. Formatting your hard drive will mean losing everything and a slim chance of getting it all back.
If it's not already there after the hard drive replacement, then it should be on the backup you made, if you made a backup, if not, swap hard drives, run the backup wizard, reswap hard drives, restore from backup
By formatting the hard drive.
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.
When you reformat a hard drive you change it to whatever formatting you have selected and it will overwrite whatever format was used before.
You can completely clear your external hard drive by formatting it.
Yes.
Low-level formatting
Yes, formatting a computer hard drive will delete the operating system.
Yes.
Use Disk Utility to format or partition hard drives. You should back up the drive before formatting or partitioning.