/dev/sdb is your drive. The drive itself is never mounted directly — or at least, not in any normal case — but instead you create partitions on the drive, and it is the partition(s) that you mount.
That’s a good thing, because you would have ruined the entire drive.
Check which partition is mounted on that drive…
mount | grep sdb
Once you know which partition it is, make sure that you don’t have any file manager open on that partition, or any program reading from that partition. Only then can you reformat it.
You may also want to read these two tutorials below…
