thanks for the support,
Yes, I've enough space in NAS (more than 500 GB available) and currently there are stored files bigger than 45 GB.
I did some other test; using Filezilla via ftp I been able to save the "offending"
50GB file from VMware ESX Server to another physical server and then, again using ftp and Filezilla uploading successfully to the final destination (DNS-323 NAS I mean).
I repeated the latest step using FlashFxp instead of filezilla, but the file transfer hangs up at the same point (about 45 GB).