Comment 9 for bug 268123

From 'man 2 mount':

       EBUSY source is already mounted. Or, it cannot be remounted read-only,
              because it still holds files open for writing. Or, it cannot be
              mounted on target because target is still busy (it is the work‐
              ing directory of some task, the mount point of another device,
              has open files, etc.). Or, it could not be unmounted because it
              is busy.

But the most typical if the block device is already in used and locked by some
[other] __kernel__ subsystem.