[I have asked the mods to move this to the "General" section, as it was posted (wrongly) in the "Manuals" section]
vineeshvv wrote:i have some old harddisks with me (80gig x 5) i want to build a storage unit
That project is not a disk repair or data recovery situation, so IMHO this forum is not likely to be the best place for you to ask.
There are many aspects to consider which you have not mentioned (e.g. do you want to use any specific OS on the storage unit; number of disk drive interfaces available; does it make sense to use 80GB disks; which protocols would you use for access; GUI needed?; backup requirements; etc. etc.). However, personally, I am not interested in these answers - they are the type of questions for you to ask yourself, as they will help you to define which solution(s) fit your requirements.
vineeshvv wrote:plz provide a step to step method to build that unit
That would take away all of the learning experiences which you will have, when you do your own project. You could look at FreeNAS, which is a popular basis for people to build their own storage systems - there are other similar software packages, depending on your level of skill, your preferred underlying OS, etc. Google is your friend.
If you cannot do this project yourself, then perhaps you should just buy a cheap commercial unit, or find a local friend who is happy to help you step by step.