Either delete and re-download movies when you start to run out of space, purchase a single-drive external storage unit of some type, or start saving up for a small NAS (my recommendation.)
http://www.synology.com is my first choice for NAS.
I checked out QNAP, NetGear, Western Digital, and Synology when I did my research last year. Although my requirements are a lot more "robust" than yours (i.e.: I'm running VMware's ESXi at home for work-related reasons.) I needed something that included support for iSCSI, NFS, and (if possible) VAAI support for the VMware virtual disks, which the Synology line does now as of v4.1.
As you begin to accumulate more movies over the years, your storage needs will grow, as you've already noticed. So, I would start with a 2-bay NAS enclosure minimum, with 2TB drives in each bay, mirrored. If you feel that 2TB won't be enough, your next option is to either get 3TB or 4TB drives, or a 4 or 5-bay NAS enclosure. A 2-bay setup will probably set you back about $500 or so, 2TB drives included (if you buy the drives separately from somewhere like NewEgg.) A 5-bay enclosure is about $900, without drives, to give you an idea of ballpark cost.
Needless to say, once people start seeing the costs involved, most turn their attention to single-drive external USB/FW/TB drive enclosures. They have their advantage in that a single drive is much more portable than a NAS enclosure ... but that's relative to the size of your library too. If it's too big to fit on a single drive, then you go out and buy a second single-drive enclosure, or start to manage multiple libraries, one per drive. Couple years down the line, you have three external drives, and you're only using the most recent/largest one, because it's too much of a pain to carry around multiple drives in your case ... true story.
With the money and time I've spent on external drives and building and maintaining DIY NAS systems made from left-over PC parts over the years, I was severely remiss in not getting a dedicated NAS unit sooner. Ah well ... that's what the benefit of hindsight gives us.