Wat is the best method to keep back-up of data in hardisk

Depending on the volume of information and how often it changes you have several options.

Small amounts of data, USB thumb Drive or CD-R
Larger amounts DVD-R or External USB HDD

There are digital tape backups for very large amounts of data.

If the data being backup up is updated/changed oftern, then a HDD or tape backup is best

It depends on how old your computer is and how big your HD is for small amounts use DVDs and the other way i do it is have a secound hard drive in the computer so if something happens to the system it usually does not effect the second hard drive Another way is to have a large USB portable hard drive I sometimes use this as I take information to another location