https://www.bleepingcomputer.com/news/security/two-thirds-of-second-hand-memory-cards-contain-data-from-previous-owners/

A recent study conducted by academics from the University of Hertfordshire in the UK has revealed that almost two-thirds of second-hand memory cards still contain remnants of personal data from previous owners.

For their study, researchers analyzed 100 second-hand SD and micro SD memory cards purchased from eBay, conventional auctions, second-hand shops, and other sources over a four-month period.