Where are their hard disks???are they under security????

No buddy it is not like that. All of them have their own private server rooms. The storage is done in huge rooms full with processing, storage and backup hardware. And as for the different hosting services, they donot have private harddrives for that, they have seperate folder for each user but you cannot access another persons folder because of the protection and firewalls and stuff

I missed some things in the above discussion. so iam providing them here. Each server is protected by 8-10 firewalls minimum. So on one can access the data without proper permissions. There will be a team working behind to protect the server 24x7. And finally the storage capacity of such big servers is calculated in Terra bytes.

i think its called mainframe server not hard disks is a storage for your computer and online storage uses mainframe server :) simple.

A server, which stores these highly secured hard drives (most if not all of which are in a RAID arrangement), offers a service to users on the Internet. Web sites, for example, are stored on web servers; transferrable files are stored on FTP servers; e-mails are stored on e-mail servers, and applications (such as a shopping cart) are stored on application servers. You can normally tell which server you are accessing by the use of the URL; you are, after all, needing a service from the Internet, such as e-mail or web page, so you need to access a particular server. Normal webpage access begin their URLS with the HTTP protocol, which is a protocol for accessing web-documents. Websites that allow you to download files directly begin with the FTP protocol, which stands for File Transfer Protocol. E-Commerce websites have recently started to use the HTTPS protocol, which is essentially an HTTP protocol with encryption security. That was just some extra information on the use of Servers on the Internet, if you were interested in wanting to know more :)