8GB Cruzer Switch problems

I have been using my USB on PC and MAC for the last few weeks and today for

some reason it’s not allowing me to save or copy  anything onto it even though it have more than 5GB available. The error message says:

…cannot save this file.Make sure that the disc you want to save the file to is not full, write protected or damaged (F:\Doc2.docx)

Can anyone tell me how to fix this please?

Return the device to the seller or contact SanDisk for a replacement. http://www.sandisk.com/about-sandisk/contact-us/