USB over current error

If the problem exists due to there being too many items on the USB bus,
or if one of the others (not camera) is drawing near the total output of
power the USB bus can provide, and if you have more than the keyboard
and mouse plugged into the computer, unplug the other peripherals.

If your computer does not already have a powered USB Hub, see about
getting one. Depending on the model of iMac (if G4) USB would either
be USB 2.0 or USB1. Not sure how backward compatible the newer
USB speed products are in this instance; but a powered USB Hub could
help if the only issue is too much demand on the computer's USB ports.

The powered USB Hub has an AC power adapter so it gets power from
the wall plug and not the computer's bus. This should help.

There may be some other issue in the USB based products you have
attached to the computer, so if the overload is due to some recent
addition or change of printer, or other add-ons, consider those as a
possible cause of the additional load.

In my computer set-up, the keyboard is directly attached via USB to
the machine, the only other USB cord from the computer goes into a
USB powered Hub. Mine is an older USB 1.1 hub, but it goes just fine.

From this Hub (a cheap unknown name with AC power adapter) one
of my printers is plugged in, and the Apple pro (black) mouse. The
only other USB item, plugged into the powered USB hub, is a data
cable to the APC UPS backup power supply. The uninterruptable
power supply is controlled by the Energy system preference panel
in the operating system, and it can shut down the computer by itself.

So, if the camera is only the tripping point of an issue involving USB
power usage, then there may be an overload in what is already
plugged into the computer; or it is very close to the USB bus' max.
The ports on the computer are powered to a certain limit, and over
that, other power sources (self-powered peripherals) are needed.

You may be able to get away with using a card reader for the Nikon.
I have one, but don't use it with my Nikon D70s; since I have three
batteries for the Nikon and they need to be used anyway. Batteries
need to be exercised, and will go bad from disuse; so I don't use a
card reader with my DSLR, nor do I with my Panasonic Lumix DMC-
FZ1. Both use different kinds of cards, and I have two card readers.

A card reader may also add a problem to the USB in that for some
card readers, the card may need to be plugged into the reader and
then the reader plugged into the computer, to be seen properly. Or,
in some cases, the reader first plugged into the computer, then add
the card to the reader's slot. Be careful, since unmounting the icon
of the card from the desktop has to be done before a connection is
changed; and damage to the card's disk image (readability) may be
a result of improperly unmounting the image or just unplugging the
card reader or camera before the icon is ejected or dragged to trash.

If a powered USB Hub does not help, the computer may need to
have the various ports and power supply (in computer) tested to
see if there is a problem; the USB ports may not be getting all the
power to original specification, or they may be overloaded.

In any event...
Good luck & happy computing! 🙂

Also, try connecting the cable to the Mac by itself. I had a USB cable that would cause an "overload" just by being plugged in, even with nothing at the other end. It was apparently shorting out inside the cable somewhere.

Even if the cable does not cause the short by itself, it would be a good idea to try another cable. There may be a problem with the connector being worn or damaged at the camera end. The problem could also be on the camera side of the connection.

Just cut your usb cable or cables, and reconnect just the green and white wires. (data) Then find or buy a wall plug power supply (wall wart) that is 5.0 volts at 3 amps or more. Cut the jack off the power supply and be sure to find the correct polarity ( + or - ) of the supply. Connect the ➕ to the red wire of the USB going to your device, and connect the ➖ to the black wire of the USB going to your device. Make sure all wires are taped and they are not touching one another. Also tape the two wires from the other end of the USB that have been cut that are not being used. (Don't let them touch either). Plug your wall wart power supply in the wall and start enjoying your devices!!!

For the cost of a new power supply (of possibly wrong ampere output)
one could probably buy an inexpensive four-port USB powered hub &
have an assurance of a limited guarantee. Or a dirt cheap USB power-
ed hub from a second hand store; making sure the hub - and a power
supply which need to match - are both present, for about $2.

{Of course, value and opportunity vary on the other side of the Pond.}

A few years ago, I bought a two-pack of new USB powered hubs for
$12. and they each had four ports. So long as the product is powered
and does not rely on guesswork to just work, even $20. is a deal...

I wouldn't expect someone with a nice computer to make a cheap part.
By now, I'm having to assume the original poster found an answer. To
have spare cables and extra components is a normal part of life, here.

In any event...
Good luck & happy computing! 🙂

{ edited }

This problem is definitely a short in the cord you are using. I had the exact problem. Everything working fine till one day I plug in a usb cord for a certain device and right away I get the USB error. I switched cords and back to normal.