Were you running the study on multiple machines or just this one? The only means by which MediaLab can delete data from your data folder is through the "clear data" button which appears at the start of a session.

For more detail, see:

http://www.empirisoft.com/medialab/h....htm#dataclear

Normally, if you clicked this button you would be given a clear warning that the data will be deleted. As of v2006, deleted data will usually go to the recycle bin if the session is located on the local hard drive. Note that there is an option in preferences to disable this feature if you are concerned that it may be used accidentally by someone else running your study.