Try another USB port.
If that fails, possibly the stick itself is faulty. You can test that by trying it in another computer.
I have been given a memory stick and had recently an opportunity to use it.
However, when I tried to save a few photos, I got a message to 'please
insert a memory stick' into the drive, which I had done.
What would be the reason that it isn't recognized?
I used a memory card instead and it worked just fine.