Is it possible to have a harddrive or sd card feed data into the Arduino to be executed and then feed more data in and so on. If it is possible how is it done?
A harddrive would be difficult to connect, but an SD card will work just fine.
You can write a sketch that reads data from an SD card. There is library support for both SD and FAT16/32 file systems.
Another interpretation of your question is; is it possible to load a sketch from an SD card. The answer is yes; there are a number of projects that have done that.
Cheers!
