Hi Jamie,
the Cache full error will be with the fact that you have something in the channel that consumes enough frames that the item your loading would need some of. What you can do is load ID "0" which will eject the channel -- or just load a still into the channel -- also make sure that the associated alpha channel is not loaded with the original items frames of alpha.
Yes when you delete them from the usb there is still an entry in the Database. if you click on it it will then ask you to confirm the removal of it from the database to free up that entry ID. Then it will be gone.
Les
#Vision