To be clear: if the stick has
ever been removed without "ejecting" it, you will probably
always get an error until it is reformatted or repaired. IOW, the act of ejecting it alone will not fix the problem.
What has worked for me in the past is either one of two options:
- Using a built-in Windows utility to repair the stick. (Can't remember how I got to it, but I think it was offered as an option when Windows detected that there was an error with the stick.)
- Transferring all of the data off of the stick, reformatting it, then moving all of the data back on to it.
I think what is happening is that when a stick is removed without ejecting it first, your computer might still be in the middle of writing to the stick, even if you are not actively transferring files. Apparently, Windows will sometimes wait to finish writing data to a drive until it feels like it. This is called "write caching" and it means that sometimes, if you remove a stick without telling Windows you are going to do so, the OS will not have finished writing to it. Hilarity ensues.
When you eject a stick, you are telling the OS to go ahead and finish everything it was doing so that nothing will be corrupted. This is the only way to be sure that it is indeed "safe to remove the device".