The BitTorrent client is set-up on an old netbook that also performs a role of local storage server.
The disk you've mentioned is an old MP3 player connected to the router as a storage device for sysconfig backup and logs. It's not a rootfs device. It's just for storage. I'm also aware of the bad disk errors in syslog but I don't worry about it since it's not a critical part of my set-up and the router can perform without this device connected anyway.
I've seen something somewhere about number "of switcher or packets on internet switch" that might cause this issue. Before I had an Realtek based switch so this might be somehow connected.
I've also seen this so I think those two problems are somehow connected: