What is the difference between "medical" grade and ordinary AC-DC power supplies?

Medical grade power supplies have supposedly been tested to specific standards. In particular, the maximum leakage current between the hot and isolated sides will be very low. There are also other internal requirements, like a large creapage distance accross the isolation barrier.

The medical-grade requirements are to make the supply more safe, even if the isolated side is connected to a human more closely than thru ordinary skin. Some of the requirements are to reduce the chance of failures, and to decrease the chance that any one failure could electrocute someone.

All this costs money, so medical grade power supplies are significantly more expensive than ordinary ones. The volumes are less, which again drives the price up.

These extra specs have nothing to do with voltage stability, ripple, current rating, and the like.

As far as I remember, a medical grade power supply is supposed to support a voltage of 2 kV for 60 seconds.

This is essential to minimize the risk of an electric shock.

Every device has to pass a risk analysis, and even more so every medical device.

The use of a medical grad power supply is an important thing to pass the necessary tests. Most medical devices are supposed to be applied on a human.

If, e. g., there is lightning, or the power grid has a problem due to whatever other reasons, and the over-voltage hits your external hard drive, it's bad luck. But if it hits your wife while applying this device, it's fatal. For this reason, it is important to have a medical grade supply.