Can't backup into a raid usb external hard disk

I have this two external hard disk connected to my pc:
a) usb hard disk
b) usb hard disk RAID
i have to do a backup from hard disk (A) into hard disk (B).
However, when the backup start acronis display only the message "calculation in progress". I wait about 2 or 3 hours but i see only this message. Why don't work ? Perhaps because the hard disk (B) is a RAID hard disk ?
Can i resolve this problem ?
Thanks