When I plug it into the USB I get the following message: "A USB device has exceeded the power limits of its hub port"

SOURCE: Western Digital Passport 160 GB external HD won't load.

WinXP SP2 blocks the use of the drive because it thinks it draws too much power. But there is a simple procedure to override this (from WD site):

"Problem:
Under Windows XP, an error message of "Power Surge on Hub Port" or "USB device has exceeded the power limits of it's hub port" is displayed when connecting a USB device to a USB port on a computer or Hub.

Cause:
Under some computers, USB devices are only allowed to draw a maximum of 500 milliamps (mA) per connection from the USB port on the computer system or from a USB hub. Under the Windows XP Operating System, with the release of Service Pack 2, if a device attempts to draw more than this (500 milliamps), a USB port balloon message is displayed to indicate this over current condition. When connecting a USB device that draws power from the USB bus, a very brief over current condition is sometimes detected by the OS, and the warning message is displayed.

Resolution:
This message has no affect on the functionality of the USB port. The USB hard drive should still function correctly on the USB port regardless of the error message. To prevent the USB Power Surge Alert message from displaying, please complete the following steps:
Click on Start.

Click on Control Panel.

Double-click on the System Icon.

Click on the Hardware Tab.

Click on the Device Manager button.

Click on the + next to Universal Serial Bus Controllers to expand the selection.

Right-click and select Properties on any listing that displays USB UNIVERSAL HOST CONTROLLER.

Click on the Advanced Tab.

Check the box that says: Don't tell me about USB errors."

This works fine, and the drive works fine afterwards. I suspect it actually draws less power than they say. Maybe it has a brief power draw spike on startup, but it's running power seems less. In any case, it works fine without the Y cable. I do think it is tacky that WD doesn't give the cable for free, since my $7 Sabrent 2.5" Hard drive enclosure came with a y cable.

But most likely the drive will work fine for you. It is a lovely quiet drive, and takes up no space at all.

SOURCE: Firelite usbflb40-c USB device exceeded power limits of hub port

I devised these simple steps that tend to remedy quite a few issues with most portable / external hard drives (though not always)
A few things to check but assumes USB and Windows for other interfaces / operating systems similar steps may be adapted to suit.

1. Ensure it is connected directly to the computer to a USB 2.0 port not a USB 1.0 port as this can have effects on performance and reliability
2. Use only the cables that came with it NOT one that fits that may have been lying around or is longer. Not all USB cables are equal even though they should be)
3. Do not connect through an external USB HUB unless that hub is USB 2.0 AND has its own power supply.
4. Use ONLY the power supply that came with it if it has an external power supply
Don’t use any other unless you know it has both the same voltage and current rating e.g. 12V 500mA anything rated below that would not work properly.
5. Always use the same port for connecting your devices. Some devices do not like being switched about. If switched they may want to install software / drivers again.
If you checked and fixed anything there and still have issues carry on reading.
Go to your control panel and then administrative tools, select Computer Management.
Now select Disk Management and expand the window.
Your drive should be listed here, if not then you need to check your computer hardware has not got any issues before proceeding.
If your drive is listed and says healthy then right click on it an select Properties, click Tools and then Error Checking, Check now, tick the two boxes and then Start.
This will attempt to fix most minor / common errors on the drive. It may also ask you to restart which is fine. Allow it to finish its work or you could corrupt the drive. If all went well you should be able to use it normally again.
If you drive was listed but did NOT say healthy then right click and select Format. Choose NTFS as the file system and do not tick any boxes you do NOT want quick format. Again start and let it do its thing. When it is done restart your computer and you should be good to go.
If you still have problems with the drive you might want to look at other areas.