You want to free up some hard disk space in your main drive. You can easily do that by turning managed master files into referenced images. You can, of course, export individual projects out of your main drive, and then add it back in from its new location, but a quicker or better solution may just be to convert managed masters into referenced images into a new location that you specify. Doing so will remove the master images from the managed Aperture library and transfer it into any connected volume that you have designated. When you move images this way, each photo will be marked with a pointer that marks it as referenced instead of managed image.

To do this, select the managed images. Go to File > Relocate Masters. A dialog box will appear that guides you which new location you want to store the photos, in the process, converting them from managed master into referenced images. The dialog box allows you to keep your pictures organized by providing a means to create a folder structure. You can also assign new name to the master images. When all is done, your photos will now have been moved away and each image thumbnail is marked with a referenced badge.

By the way, you can also use the Relocate Masters to move photos which you originally referenced on import from one to another location. This is a little-known but very neat Aperture tool that provides you with safe and flexible means of moving your images. Of course, as in any photo management system, you must decide for yourself and device your own logical strategy for organizing and handling both managed and referenced libraries in Aperture, as well as images out of Aperture.