Steve - thats been a random bug since ICS and maybe even before. Try some file maintenance on your SD card. Delete the Lost.dir on your internal SD, and the .thumbnails folder in your DCIM folder. Also, maybe try backing up your important data from both the internal and external sd, format them both, then load your data back onto it. That problem always seemed to stem from cache files. I personally have never seen it, but I run basic file maintenance before every flash, it takes just a few minutes, and saves a ton of headaches.
You ride yourselves over the fields
And you make all your animal deals
But your wise men don't know how it feels
To be thick, as a Br1cK
- sign up and we both get free space!
only if you really feel its necessary.