A write protection system 202 is provided for machine-readable data storage disks, such as optical disks, of a single or multiple disk cartridge 100, where write protection of any disk can be individually enabled or disabled while the cartridge is inserted into a disk drive. Separate trays 104, 108 support each of the disks, all trays being housed by a cartridge. When the cartridge is inserted into the disk drive, only a face 103 of the cartridge is exposed. Each tray includes a write protection mechanism positioned near the cartridge face when that tray is housed by the cartridge. Each mechanism provides a user-selectable, machine-readable indication of the protection status of its associated disk. An exemplary mechanism includes a resilient finger 204, deformably pivotable from a released position to an activated position when a deflection pin is pressed against the finger through an activation hole in the cartridge face. A detente 216 is resiliently urged toward the finger, at an intersecting angle thereto. The detente includes a ramp 218 with a catch 220 facing the finger. As the finger moves from its released position to its activated position, the ramp slidably contacts the finger's tip. The catch maintains the finger in its activated position after the tip slides past the catch. Manually extending a deflecting pin against the detente through a releasing hole in the face pivots the detente away from the finger, causing the tip and the catch to move apart, whereupon the finger's resiliency snaps it back into its released position.