I have used an SD Card to act as internal storage and have had to format it 3 times now in the space of probably 3 months - including all the bother of having to go and re-install the apps that I had stored on that SD Card.
There is no reason to think that it is an SD Card issue. It works fine most of the time and then suddenly It fails to recognise the card or believes it is corrupted and needs to be re-formatted.
Are there any known issues with charging or rebooting phone that could be impacting the SD Card recognition?