SD card won't format on my Samsung Galaxy S3
If your device talks about "low memory", it is not evaluating the sd card -- but rather means internal memory. So it doesn't help to buy a bigger sd card: what you have to do is to free up internal storage.

This is an issue coming up here frequently -- so for details please look at the solutions already present:


Low disk storage Space
More internal memory or less memory + SD card
My internal storage capacity is running low, what can I do?
Message Memory Full, Incoming message rejected


There are many more -- just follow the internal-storage tag and watch out...