Copy of contents into PR4100 stops unexpectedly

Hello to everybody.

I have a new PR4100, firmware 5.18.117, using it with a Mac (High Sierra 10.13.6).
After some Gbs copied I get the message: “finder can’t complete the operation because some data in FILENAME could not be read or written (error code -36)”

If I then try to open the volume on finder, I get some “Impossible to find the original element”.

After a while, the volume is visible again, but the copy is interrupted. Any ideas, please?

Pinuccio