Data loss on external hard drive

Dear experts,
in the office, my girlfriend saved two Excel sheets on her brand new pocket size 2.5" external hard drive, which was connected to the office computer via two USB plugs.
The office computer is quite recent, has Windows Vista and Office 2007 installed.
The external hdd is a Hitachi 500GB inside of a Fantec case. We screwed them together ourselves but it was very straightforward.

The point is that when my gf arrived home and wanted to open the files on her Sony Vaio notebook, at least one of the two files showed up, but when she tried to open it, there were only strange characters in Excel - nothing readable. She disconnected the drive safely and reconnected it, and then, both files were lost.

I managed to save one of the files using "test disk" 's undelete feature. In fact, I couldn't recover the file that was lost, but a temporary file that Excel had saved. The fact that Excel had put that temporary on the external hdd indicates that she really run the file from that drive, since Excel usually saves temp files in the same folder as the actual file.

Questions:
1. What happened? How can that happen?
2. Can it be recovered? How?
3. How can that be avoided in future?