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OT (most likely) strange modem disconnection when using USB drives

I'm pretty sure this isn't Fedora related, other than I'm doing this on
Fedora, and perhaps there's some diagnosis tools I can use with Fedora
to look at the problem.  Perhaps someone on here might have come across
this before, and I've had no luck googling it:

I have ADSL, the modem/router's in another room, it's always on, all the
computers connect to it via UTP ethernet at 100 Mb/sec.  If I plug a USB
hard drive enclosure into a PC elsewhere on the network, the modem
disconnects within seconds.  This is repeatable, without fail.  All the
PCs are running some version of Fedora, but they don't even have to be
running to upset the modem, I can kill it by plugging it into a box
that's halted.  Other than this, the modem will stay connected for weeks
without any issues.

This is odd, because theoretically I could do anything to this PC, run
it through with a chainsaw, set it on fire, etc., and the modem would
never know, nor disconnect, unless I zapped voltage up the cabling.
You'd have to telnet to the modem, or visit its webserver, and log-in to
alter what it's doing.  There is no connection between USB and UTP
ethernet, theoretically UTP ethernet might even be electrically isolated
from the rest of the box.

I can even disconnect a PC from the ethernet, connect the USB enclosure
into this isolated box, and kill off the modem connection.

The only other example I know of USB killing a modem connection is with
a friend, he has a similar story:  Seconds after starting to burn a DVD
on DVD burner in an external USB, his ADSL modem disconnects, too.  His
is a different model, same brand (Billion), and the networking is the
same (UTP ethernet).  He has other USB enclosed drives, and they don't
exhibit the same problem.

His system was Windows, so I gave up trying to sort out the problem with
his software, I'm just not familiar enough with it to try.  Though I did
dig out my CRO, and went probing about to check for strange signals
(oscillations on broken grounding, stray interference, etc), and found

My only guess at this issue is that the USB enclosures are radiating
some crap that really upsets ADSL.  But re-routing ethernet, USB, and
phone line cabling, adding more shielding, etc., doesn't change
anything.  I'm looking for something new to try.

Yes, I took the USB enclosure back, swapped it for another brand.  The
same fault exhibits (connecting it disconnects the modem), but not quite
as bad as the other (the modem loses connection periodically, but not
quite as often as the prior USB enclosure caused - but it's still every
few minutes apart, though you might get a good half hour without

The USB enclosure uses a switch-mode power supplies, the modem uses a
simple transformer plug-pack.  My next step is to swap the modem power
over to one of the lab bench supplies.  The phone line is away from the
ethernet line, except for about two metres where the cables drop
together into the next building, and that'd be a real pain to separate.
Anybody think of other things to try?

(This box runs FC5, my others run FC4 & FC6, in case that's
 important to the thread.)

Don't send private replies to my address, the mailbox is ignored.
I read messages from the public lists.

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