This is an error which I have seen many times, and yet while there are multiple questions about it (here and here), none of them are up voted. Is this because there is no answer, or I can't search and find the accepted one?
It appears to be a bug.
In both cases the suggested fix is the same (to remount / rw), so it is probably just two different errors messages (one each for / and /tmp) representing the same underlying problem.
edit: Also Disk drive boot problem: The disk drive for /tmp is not ready yet or is not present! which had answer:
This is a bug in Ubuntu - Ubuntu bug #1091792. It was fixed in mountall-2.48 for Ubuntu 13.04. Unfortunately the latest version in Ubuntu 12.04 is mountall-2.36.4, so it is not fixed (and, presumably, never will be).