These machines typically use a 2.5" hard drive. If you are lucky enough to have one of these, it's not an issue to replace the hard drive. With that being said I've heard the SSD only models don't have the SATA interface; I'd tear these down and make sure if you have one. It's difficult to open since you need to take the motherboard out to access the RAM and SATA hard drive, but it can be done
While 2.5" models can accept a hard drive, the whole system has to come apart. This is not only true for the hard drive, but also the memory. These ZG5's don't have any access panels. The guide to open the laptop can be found here.