USB Disk Stalls on -current

I'm doing something "gross" with ZFS & Plex on a little Intel NUC that I 
have here at the house to provide me with a nice little NAS at home. 
I'm using 2x USB2 external disks as the mirror.

I noted that the two USB2 disks I'm using in a mirror seem to "stall" 
from time to time and its not clear to me why.

I'd like to poke further into the USB system but I'm not sure where I 
should start to see if there is something amiss with the hardware (e.g. 
the disks suck) or if FreeBSD is losing track of something during I/O 
leading to a stall/timeout.

I'm not seeing data loss or anything, I just note from time to time 
during large file transfers that the clanking/grinding sound of the 
spinning rust on my desk completely stops, the encoding of the video 
files stops (so its waiting for a read to complete) and its gets much 
quieter in my office.  :-)

sean