Why are my SD cards playing up?

I have a number of SD cards that I have not used for about a year. On three of them the camera screen said ‘no valid data’ so I presumed they were empty. However, when I tried to take a photo, the message was ‘memory card full’. The cards were unlocked and the same message appeared on my second camera. Is this a common problem?

Hilary Kemmett, by email

Good quality SD cards, and flash memory in general, is extremely rugged and should be able to withstand 100,000 or more read/write cycles and still be readable after 5 – 10 years of non-use. Data is also safe from low-level and moderately strong magnetic fields and their only real enemies are extreme temperatures, very high levels of ionising radiation and physical damage. The chances are the duds were either cheap rejects, counterfeits or fakes, where low capacity cards have been hacked to display a higher capacity. You can check for the latter with a free utility called H2testw from http://goo.gl/HnlQ8M