An 'invalid pack' in the da driver means that the media for the drive is
temproarily missing, pending an action by a third party (eg robot arm or
human inserting a disk pack, etc). We invalidate a pack when the
ASC/ASCQ tells us the media is missing (ASC 0x3a MEDIA MISSING) and for
that brief period of time after a peripheral has been invalidated before
the disk is destroyed. When the pack is invalid, we can't do READ/WRITE
operations to the media, so we fail all outstanding I/O and any new I/O
that comes in when this happens. This is cleared in daopen when we've
repobed the device.
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