I am loosing my downloads when i restart DC after a few days

[DrRex]

Hi there, can anyone tell me how it comes that, sometimes when i start DC after a few days of non using, that some of my partial downloads (not all !!) are gone ?
It's very frustrating when you downloaded 1Gb of a movie and it's all gone and have to start all over again. I' am using an external hard disc, can that be a cause of the problem?