I was using Sigil 2.0 beta Mac version, and couldn't get it to save anything on either of my macs. I dit what Tlrowley suggested, made sure that all my images and files had no spaces in their names, and this didn't solve the problem.
I'm creating the epub files from InDesign CS4. I'm using Sigil to do post formatting and editing of the generated CSS.
When I changed to version 1.9 of Sigil, the file saved correctly and with no problems. I'm thinking that V2.0 still needs some work.