getimagesize warnings...

I'm trying to upload images to the websever I create a temp file and copy it to the webserver 95% of the time I have no problems, occasionally I get the follwoing errors.. Warning: getimagesize: Read error! When I check the web server a file with the same name of the image I tried to copy exists, but it is 0KB. If I delete some unused files from the server and delete this 0KB file the process works fine. Hence I naturally thought it is a space issue on the server. But there is 20% capacity used on the web server!! (running Apache on a unix machine) Any ideas? Thanks.