What kind of data can be stored on the blockchain ?

[Evil-Knievel]

I completely agree with you, but I think the OP just wants to know if he can use a blockchain to store any form of data. I believe the answer is that blockchain technology can be used to store any form of digital data but of course each time that data has to be accessed, it's a transaction and it costs money and network resources

So every time I access the blockchain files on my harddrive I have to do a transaction?

[Evil-Knievel]

You're not the only one who think the same most of people in this forum are thinking the same way but let me remind them blockchain isn't a database.Once you put something inside on it you'll never able to re-write it.

You won't be able to delete it, but you can certainly rewrite it. What would prevent you from doing so? You just need to figure out a way to mark old entries "dirty" when updating it - maybe with a reference to the block height where the old value was stored?