Airport disk won't let Windows 7 copy ...

Dear readers,

I've got an Airport Extreme, with a PC (win7x64), Mac mini and USB disk (Lacie Porsche design 500Gb) connected (wired Gbit).
To get acces to the disk I already changed the Windows settings below. Now when I copy several files or 1 big file to the Airport-disk, Windows pops a message that the connection is lost (although I still see the disk in Explorer). I can chosse "skip" to skip the first file, after which the rest is copied perfectly.
The Synchronize tool "SyncToy" has no problem copiing any file to the disk.

So it seems Explorer has a problem initiating a connection needed for copiing in a first attempt, right? Perhaps SyncToy is more patient and waiting for a good connection?
Does anybody have a idea how to solve this?

Thank you


security setting change, needed to make the disk visible:

Go to Start Menu
Search for: security
Click on Local Security Policy
Goto Local Policies

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