K6 on thumb drive often crashes when another USB plugged in

If I run Knop601 from a USB drive, it runs fine for days - even looping md5sum the KNOPPIX file is fine -
until I plug in another USB drive. Sometimes it is ok, but often Knoppix crashes.

The light on the Knoppix thumb drive goes out and stays out like it is not plugged in.
(K511 and K531 have also crashed often when running from a thumb drive)
The relative positions of the Knoppix USB drive and the newly plugged drive may be important.
(I have not written to the thumb drive or made an image file.)

If nothing is running I can sometimes use the power button to force a shutdown:
Running OO, it hard locked. Running terminal any command just says input output error.

Background: While running K531 from the internal HD, I have noticed dmesg showing USB disconnects followed immediately by detecting the same device. It is often mounted somewhere else - e.g.
/dev/sda1 -> /dev/sdb1
WD passport USB HD did this often, WD mybook HD never did.
(A Flash Voyager thumb drive did this too.)

This happened on 2 different computers with 2 different passport drives.
(the same WD passport works fine on K6 if you don't plug another USB in)
-- edit --
Workaround:
Plug a USB2 hub with its own power before you boot.
If I only plug into the hub after booting I have not had any trouble.