Why is e2fsck aborting due to /dev/sdb3 being currently in use?

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e2fsck is detecting that the file system on /dev/sdb3 is currently being used by another process or application. This may be caused by open files, active system processes or running applications that have files or data stored on the file system. In order for e2fsck to check and repair the file system, it needs exclusive access to it, which is not possible if it is currently in use. To run e2fsck successfully, the file system needs to be unmounted or the system needs to be booted into a recovery mode where the file system is not in use.