A loose cable or an otherwise intermittent connection can sometimes
cause this. The drive doesn't fully deregister from the system, but it
reconnects and the system thinks it is still mounted and tries to mount
it again, naturally failing. If the cable is secure on both ends, and if
another cable doesn't solve the problem, it is likely that the drive may
be going bad, especially if it's a spinning drive. Of course an
underpowered spinning drive can also cause this type of problem, as the
power going to the motor is good enough most of the time, but is not
enough other times, causing a soft reset, which would cause the error
you mention.
~Kyle
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