Junos MX5 error after OS upgrade: System is running on alternate media device (/dev/da1s1a)

After OS upgrade on Junos MX5, you are hit with following notice upon reboot;
— JUNOS 13.3R9.13 built 2016-03-01 07:16:35 UTC
NOTICE: System is running on alternate media device (/dev/da1s1a)

This error is as a result of various scenarios and below are the possible causes:
1.) A software failure/error on the primary media (flash)
2.) A hardware failure on the primary media (flash)
3.) A request issue reboot on the secondary media (disk)

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