I search on ebay , and find that i can buy an internal HDD and convert it to become as an external one, also i found this :
http://cgi.ebay.com/2TB-Silver-USB-D...item3cac5e9669

its an enclosure that act as a raid and it can take up to 2Tb at the same time .
so you can buy like this solution , and buy as many internal HDD ( that you use a different HDD for each day ).