When a memory card is unreadable or reports a fault, work through the following.

• Card type: continuous 24/7 surveillance writing requires a high-endurance surveillance-grade card. Consumer cards exhaust their write endurance quickly and then present as faulty or unrecognised.
• Format and initialisation: a new card must be formatted in the device with the correct file system. A card formatted on a PC may not be recognised at all.
• Capacity and overwrite: once full, the card should overwrite cyclically. If recordings stop accumulating, check that loop recording is actually enabled.
• Readable on a PC but not in the device: scan the card for bad blocks on a PC, then reformat it in the device. If it still fails, the card itself is likely worn out.
• Power and concurrency: unstable supply causes write errors and corrupts the file system. Treat on-board storage as a supplement and keep important footage on an NVR.

Check card write status every six to twelve months, replace at the first sign of repeated warnings, and keep a spare card on hand.