Zencastr saves a local backup of each recording on a portion of the hard drive allotted for web browser applications. There are some restrictions on how big this portion is. In general, 10 - 20% of your free disk space will be made available for zencastr's audio backups.
I recommend that you have at least 20GB free on your computer as that will guarantee that you will have 2GB available for Zencastr files.
If you are using Chrome, you will be able to see in your dashboard how much space is available and how much you have left. Firefox doesn't support this feature just yet.