More USB woes

[Lennart Sorensen]

On Fri, Mar 26, 2010 at 10:49:34AM -0400, Thomas Milne wrote:
> Something weird is going on, that's for sure.
> 
> The drive would not mount at all the other night, I was getting errors
> about 'cannot read partition table'. So I switched to a different port
> and it worked fine (well, slow, but no errors).
> 
> Also, no matter what port it's connected to, if I try to burn any
> files from the drive to a data DVD, I get an estimate of more than an
> hour to complete the burn. What??
> 
> ...and just now, when I plugged in a USB printer to another port, the
> drive disconnected and then reconnected (??).
> 
> I think my computer is having problems as well...everything seems to
> happen at once sometimes.

Maybe your USB ports are underpowered.

Some computers support powering on the computer by the keyboard and
if the keyboard is USB then the USB port has to be powered by standby
voltage (5Vsb rail).  Many power supplies only had 1A available on that
rail, which limits the number of USB devices that could really handle.
Mice and keyboards are trivial since they use almost no power.  If a
USB drive is connected to one of those ports suddenly it is running on
the standby power rail and could actually overload it.

I think newer PCs only run the port on standby voltage when off, and
switch to normal power later, but I wouldn't be surprised older machines
didn't do that (after all USB drives were not as common 7 or 8 years ago).

A good power supply can usually do 2 or 3A on the standby rail, which
should handle at least 6 USB ports fully.  A good motherboard should
deal with it too.  My newer Asus boards have jumpers for each pair of
USB ports to select whether that port should use the 5Vsb rail or the
regular 5V rail.  If you set it for regular, then you can't powerup
the system using any device on that port, but you generally only need a
couple of ports set up for that.  You can of course also use those ports
to charge your ipod while the computer is off that way.  My laptop even
has a BIOS setting to select if the USB port should have power when the
system is off or not.

-- 
Len Sorensen
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