I have removed the hard drive from my old PC and put it into an external case.
When I connect the drive to my new PC it works fine, BUT it somehow causes the ADSL to stop working.
Any other device plugged into the USB port works fine with no problems.
Can anybody help, please?

I would suspect the external drive enclosure is faulty. Does the drive enclosure have its own power source?

How do you connect to the Internet? Is the ADSL modem a USB device, or are you going through a USB router?

What else is being driven by that USB port?

If you have a different pair of USB ports you might try putting your network on its own pair.

Do note you can place too big of a demand on the port by using too many devices that are powered through the port. You might considered a USB Hub w/ext. power supply.

The drive enclosure has its own power source.
I connect to the internet via a modem/router which is connected to an ethernet cable to my PC (my wife also has a PC connected in the same way). So it's not a USB device.
The only other USB devices connected are the printer and keyboard/mouse.
I've tried using another port at the back for the drive - same problem.

Thanks for helping,

Hmmm, that is odd. And if you connect the USB enclosure to a USB port on your wife's PC, what happens?

Good thinking. I wonder why I never thought of that...
It works OK on hers and then eventually the ADSL fails (on mine it fails as soon as I access the drive). That suggests that its the drive or case ?
The case came via Ebay from Hong Kong, so it was very cheap. But is it likely to be the case and not the drive? I acn easily buy another case.

Yes. That is, it could be the drive or the case. Or both - though not likely both. But I sure would suspect the case, or something to do with the case, such as the power block, or cable. I would probably pull the drive and connect it internally as a slave (assuming IDE) and see what happens. If you try this, remember to power down the computer AND unplug the computer from the wall first. And make sure you observe proper ESD precautions and ensure you and the case are at the same "potential" by touching bare metal of the case to discharge any static in your body before and frequently while digging around inside.

Thanks for the advice. I'm not confident messing around in the box, so I think that I will try another case. I can get one for about 10 pounds.
If this doesn't solve it then I will come back again.

Sounds good - keep us posted.

I can conclude on this:

I changed the case and still had the problem. But a new drive is fine. So there is obviously a drive fault.

But I am mistified as to the link between an external drive and ADSL!!

Thanks