As cheap as HDDs are nowadays, there is no need to compress a HDD - unless you are DEAD broke and have NO other option. There is a risk of compressing a HDD and depending the age/quality of the HDD, you will risk losing data.
If you have some spare change, get a 2nd hdd and use it as a slave. It's summer time here and I've seen plenty of garage sales with old PCs you could salvage a used HDD for cheap.
