I keep the Aperture app on the internal drives of my iMac and my Macbook Air. It's less than 1GB in size.
My big permanent Aperture library is on my iMac. But I keep one of my Aperture libraries on a portable external drive. I use this library for selected current projects. Having it on an external drive means
- I can move my current library from my iMac to my Macbook Air and back easily.
- I can work on current projects on the MBA without having to worry about the size of the library.
Using the library on the external disk works just fine. To open the library, I just find it on the disk and double click it.
The files in my big permanent library are referenced. The files in the portable library are managed by Aperture. Just makes it easier to move those files later on into the big library.
The main problem with the external drive library is that it's not automatically backed up by my Time Machine. So I have to be careful to back it up manually.