is it possible to change the exisiting hard drive for a larger one? 

Gonna have to guess which head unit you're talking about I think. Give us a clue?

Sorry it's a Columbus mib1 

I'd be very surprised if there is a traditional hard drive fitted, it's much more likely to be flash chips directly soldered onto the mainboard.

Yeah I was kinda hoping it was a ssd small drive or a replaceable little add on board. It seems v small capacity by today's standards. Guess I've been spoilt and will just have to make do with the sd cards. If I'm honest they will be  big enough.