Sometimes when you try to import images into Aperture, an error dialog indicating that more disk space is needed occurs, although there is more than the indicated amount of space available.
Aperture requires that a certain amount of unused disk space be kept in reserve. You may find situations where this dialog comes up when there are several hundred megabytes free, even after accounting for the space required for the images you wish to import. It's good practice to keep at least 1 GB free on the volume where the Aperture library is stored.