If you get the DISK TO FULL error it means your computer's hard drive is too full so the recorder can't record.
By default the recorder will save data to your main hard disk which is usually C: on windows or the "Macintosh HD" disk on Mac. You need to delete some files on your computers hard disk to continue recording.
If you've created many recordings then you might want to delete some of these previous ones that you no longer need. Just make sure after deleting them you restart the app since it won't actually delete the files until you close it.
If you have some recordings that you don't want to delete you should consider exporting them to a compressed .som file by clicking the blue "Export" button, then deleting the original. Later you can import the recording if you need to edit or upload it again.
If you have a Pro Account you can also choose a different folder to save your recordings as described in this video: http://help.screencast-o-matic.com/v2-manage-recordings