What is a "valid volume label"?

Disconnected my ext USB gang thing while creating an image because I have three or four USB flash drives connected and did not want the image to get confused. [in hindsight, probably a mistake]

Now, all three USD flash drives show as (OS). I keep trying to change the labels to ‘pics’ and ‘files’ and ‘enc’ [for encrypted files] like they used to be labeled, but it won’t let me.