In the past, I’ve had to simulate cases wherein a drive (or a folder) was nearly or already full. To do so, one would have to load several files onto the drive but that would be quite tedious if you’ve got gigabytes of free space. So I either used the trusty 512mb thumb drive (from a previous project) or the very small partition I created on my machine.
Another alternative was shared by an officemate earlier just in case you don’t have a thumb drive, or if you don’t want to mess around with your disc’s partitions. The tool is called TrueCrypt. It allows you to create a volume with your preferred capacity. After creating it, you can mount it and then you’d be able to copy or save files onto it just as you would with any normal disc