Some questions about da0 on USB2 (recent bad behaviour)

[b. f.]

>That is: it seems to work fine for some fraction of a minute
>(doesn't seem to be longer than a minute, anyway), and then
>stops completely for several minutes (processes reading or
>writing sit in "D" state in ps) and then starts again, after
>logging "Request completed with CAM_REQ_CMP_ERR\nRetrying
>Command".

In the past week or so, Alexander Motin (mav at FreeBSD.org) and Andrew
Thompson (thompsa at FreeBSD.org) have made a number of related changes
to cam and usb in the P4 repository, and in 9-CURRENT.  Some of these
may address your problem.  I'm not sure when they will be back-ported
to 8.X.  You may wish to try out the latest version of -CURRENT, to
see if it solves your problem(s); or to contact them.

b.