[Short Hand]

You should always keep it on.


Why ?

I will explain.

Your hardrive is constantly rewriting information all over your drive. It rewrites this information in order to prevent magnetic corruption of the data. THE DATA IS STORED ON tracks within sectors on the hard drive platter. Thus why after constant writing, and rewriting of your files over and OVER...... you need to defragment your hardrive. It is reccommended for optimal HD speed, that it should be defragmented once every day. So keeping your computer on is actaully one of the best ways to prevent data loss itself.

Magnetics play a key role in data degregation. So if your computer is off, and not rewriting data.. then data can be lost and corrupted over time. It happens really easily as well. You may not notice, but it is happening. Speakers, and all sorts of things will cause your drive trouble if your computer is off. Also, the sudden surge of power from powering on the computer also has a slight damaging effect.