Using external hard drive to edit imports blank photos

[meli8888]

Hi. I'm not sure what is going on, so any help would be greatly appreciated. I have a macbook pro. I am using imovie to edit. Everything is working fine with my internal hard drive and imovie. My internal hard drive is pretty full though, so the current project I am working on I set to save on my external Western Digital My Book 500 GB hard drive. When I import photos they show up as they are importing, but once the import is done, the photos show as black in the project bin and timeline. Video imports fine. I could just make the project on my internal drive and once it is done, drag it to the external drive, but if anyone can help me just edit the whole time on the external drive, that would be ideal.