Aperture Master File Issue Expert Help Needed

Hi,

Bought Aperture 3 about 2 weeks ago and decided to pull all my images from my external drive into aperture whilst leaving the master files in their current location on the external drive.
I wanted to tidy up my entire collection (editing master file names and renaming folders, tagging faces & places, adding other tags) then I was relocating the edited folders into new folders on the same wireless drive (making sure to move the master as well at this point).
Once I'd edited a folder I would remove it from Aperture and move on to the next but what I found was if I closed aperture then re-opened it (say after a re-boot for example) the as yet unedited folders would still be in Aperture but if I tried to move them after editing I'd get the message 'unavailable master files cannot be relocated'?
Why am I getting this message after reloading Aperture?
The external drive has not been moved to another port or location on the network (it's a network drive). It's connected to my mac as usual. All I need to do is close Aperture and re-open it and the file/folder can no longer can be moved?

welshred