Bug#600501: Acknowledgement ([linux-image-amd64] kernel NULL pointer dereference causes USB storage devices from working)
With some more googling I found this message:
So I have done:
moprobe -r ses enclosure
before plugging the hard disk, and during the 5 times I have done this
the bug has not manifested itself.
So this seems like a good workaround until/if the problem is fixed.
Probably the data structures from ses.ko or enclosure.ko are not being
cleaned (or free'ed then alloc'ed), and that causes some bad
initialization with this particular external HDD.