It seems that the smsc95xx module can not be kept responsible for
the
bad performance of your drive.
The opposite seems to be true - activity in the smsc95xx module
*improves* performance. It still smells like a missing interrupt (or
poll) problem, but I still don't know where...
Other points that might investigation,
include the USB-controller on your PandaBoard (ehci, ohci, ...) or
[ 1833.619598] usb 1-1.3: new full speed USB device using ehci-omap and address 6
[ 2318.736450] usb 1-1.3: new high speed USB device using ehci-omap and address 7
[ 2695.448394] usb 1-1.3: new full speed USB device using ehci-omap and address 8
ehci-omap is built in to the kernel.