USB memory stick - read-only problem all of a sudden

OS: Ubuntu 15.10
CPU: 64bit

PROBLEM

I used a memory stick to transfer my ssh files. I copied a directory with the files onto the stick, then took the stick out. Since then on, it was not possible either to create new files and directories or delete existing ones (even though the menu showed the action as a possibility) on the stick, or to change their permissions.  The message I got was that I was dealing with read-only file system.

SOLUTION

After various attempts to remedy the situation, I decided to reformat the memory stick. The stick originally came with some files, that I wanted to keep. There was also some old directory of mine and the new one I added last. I created a backup directory on my Desktop and tried to copy over the files I wanted to keep. All were copied successfully, apart from the last one, which, on attempt to copy, gave an error about incorrect input/output.

I concluded the problem was caused by me taking out the memory stick after adding the ssh directory without ejecting the drive first.  

I reformated the stick (fat32), copied back the original files and directories, added the ssh directory, then ejected the stick. The stick is writeable and all files and directories are healthy.