Or does it happen with reads too?
With both the sata drive (sda) and flash drive (sdb) (both usb)
connected, and network (also usb) up and running, I did some raw read
tests:
dd if=/dev/sda1 of=/dev/null bs=4096 count=102400
dd if=/dev/sdb1 of=/dev/null bs=4096 count=102400
sda 9.4 MB/sec
sda+ping 23.4
sdb 7.8
sdb+ping 21.3
both 14.7 each (29.4 total)
both+ping 14.0 each (28.0 total)
So, the problem seems to happen if you have one usb-storage active and
no other usb activity, but other activity (either another usb-storage
or ethernet pings) seems to work around the problem.