SOLVED ESXi 6.5 keeps reverting to version 6.0

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Sep 10, 2013
This is a PSA.
I had originally installed ESXi onto a USB drive a few years ago (I think I started with 5.0), and have been happily upgrading ESXi on that drive as new versions came out (using esxcli command-line - it's SO EASY) and life was very good. Until ESXi 6.5.
The 6.0 to 6.5 in-place upgrade was smooth and easy as usual, and the server happily served ESXi 6.5 until I had an extended power outage one day. After a cold boot, I discovered that my ESXi instance had reverted to 6.0, and GRUB did not see any traces of 6.5! I was puzzled, but upgraded again (because it's so easy!). Rinse and repeat another 3-4 times over the source of a year.
I got fed up with this yesterday, and decided to nuke it and do a clean install from CD. Lo-and-behold, the ESXi CD installer didn't offer my USB drive as a destination for the install.

It turns out that I had used a good-for-its-day 8GB USB drive for the original ESXi install, and ESXi 6.5 no longer supports being installed on drives smaller than 16GB, even though the whole install STILL only takes about 4Gb of space, and leaves the rest of the drive unpartitioned. What a waste!

As soon as I put a 32GB drive in, ESXi happily installed onto it, and this finally resolved the issue of ESXi downgrading itself mysteriously.

TL;DR. Check and see if the USB drive you are using is under 16GB in size. ESXi 6.5 needs 16+ GB drives to be happy.
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