while playing, open an xterm and type

"lsof | grep usb". The bar might be hard to find but it is on the symbols list somewhere.

It will list all applications that have a file opened on the USB.

metalayer-crawler WILL attempt to index it, but is faster after the first time.

Also "dmesg" might show something like using usb1.1 instead of usb 2.0 (you probably need a powered hub and it needs to be USB 2.0 - things will connect with usb1.1, but it will be slow).

Assuming there is nothing else interfering, an external hard disk should transfer as fast if not faster than the internal mmc cards.