From ComputerWeekly.com:
Research by BT, the University of Glamorgan in Wales and Edith Cowan University in Australia, has found that while 41% of the disks were unreadable, 20% contained sufficient information to identify individuals.
The research, based on the acquisition of 300 PCs from auctions, computer fairs and on-line purchases, also found that 5% of the machines held commercial information on organisations, and that 5% held “illicit data”.