Mine is this[1] write blocker. Often use with ddrescue[2] for data recovery from failing drives. For times when I don't have the write blocker on hand, I use Aaron Burghardt's handy Disk Arbitrator[3] to easily block mounting of new volumes or mount them as read-only.

[1] http://www.wiebetech.com/products/Forensic_ComboDock_v5.php [2] http://www.gnu.org/software/ddrescue/ddrescue.html [3] https://github.com/aburgh/Disk-Arbitrator