Hi there
I do a LOT of torrent downloading so this has been the most heavily used feature of the router I have used so far. It does the job really well, I can just queue up my torrent to download to a 16gb USB pen i have attached onto the router USB port. My speed from my ISP is 10mbit download, on checking the dwonload speeds for the router I have seen it max out at 1.1mb/sec download, the only thing limiting you would be the write speed of the USB pen, providing its fast enough you wont have any issues.
When its done downloading I can browse through the Network Neighbourhood in Windows and pick up the shared drive on the router and copy my files off, these copy off at up to 6mb/sec across WIFI. I have torrents on the go all the time and havent noticed this affect my internet browsing speed as I have set up QoS on the router to prioritise the traffic on the port being used as torrents as low, http is medium and real time stuff is high proriity. I also use a Dreambox for TV and stream live video across a 270mb link provided by the router downstairs up into my bedroom, it is faultless at handiling the torrent and dreambox traffic - the QoS would be the main reason for this... I used to suffer regular issues with bandwidth on my old DD-WRT equipped WRT54GS router and useless QoS. This router is a light year ahead of it.
I have noticed the router lose WIFI connectivity when transferring very large files (over approx 2.5gb) across WIFI, but i dont know if this is a network issue for me, i need to test a little more as i only have the router a couple of weeks.
The only thing I dont like about it is the router allocates double the amount of space on your download while your torrent downloads, so a 6gb torrent becomes 12gb and so forth, so I'm limited to arounfd 7.5gb with my 16gb pen. But I learned to live with that, I doubt i would change to DD-WRT on this router and loose the torrent functionality, I cant do without it now!
External hard drives should be OK with this providing they arent the type with two USB leads to povide the juice as there is only one USB port on the router.
Any more questions let me know and I'll get back to you.