Why would an internal hard drive suddenly short circuit when attached on another PC?

Solution 1:

Did you plug in the drive's power connector while the host PC was on? If so, that's not to be encouraged anyway.

The spark might just have been due to the drive startup current punching through some grime on the connector - especially if you were plugging the drive with the power on.

If, however, the spark occurred when you switched on the power then that's another thing - although I have seen this occur when a drive (Molex) power connector has been attached the wrong way round - it's surprisingly easy to do this with some of the 'softer' plastic plugs - and unfortunately this usually kills the drive. I managed to do this a few weeks back with a SATA-Molex power adaptor - note the fried chip in the pic:

The only way to find out if anything's damaged is to fire up the drive, having checked the power connector orientation and looked for visual damage (and burnt smells/fried chips) on the drive circuit board first.