I Start up the VM and on my machine Acronis boots up. This effectively connects the external USB hard drive to the VM. Now before starting the VM up I go into the USB secion of settings for the VM, and make sure 'Enable USB controller' is ticked together with 'Enable USB 2.0 (EHCI) Conroller' I now add a new USB filter - Virtual box then displays the name of my USB harddrive (A Western Digital) and then make sure the entry under USB Device filters for my newly added drive is also ticked.
I have a TIB file on my external USB hard drive - so I plug that in and let the host PC discover and mount it so that I can see all the drives in My Computer. I have insered this as a CD ROM so that when the VM boots it does so into the Acronis recovery environment. This file resides on my system harddrive. I have a copy of the Acronis boot CD which I converted to an ISO file. I have just tried out the following that may be of some help:
