I have a situation where I backup the files on one external disk to a destination on another external disk. When the two disks are connected to different USB hubs the backup does not complete. However if I connect them to the same USB hub the backup completes. Is there a way to regulate the backup speed or any other way to do this so that there would be no problem reading-writing on the external disks when they are on different USB hubs?

The devices are 2.5 inches and 3.5 inches drives respectively.