Similar issues seen while upgrading via VUM or ISO image on the hosts that have RDMs mapped. Can you try upgrading using the offline bundle via the command line?
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We have the same issue with UCS B200. While trying to upgrade the ESXi 5.5 to 6.5 through update manager, it got stuck on "About to install". However, it is going through manual upgrade with ISO mount. We have 104 ESXi host with 5.5 so doing manual upgrade will be time consuming and not recommended.
This issue is noticed if the host is mapped with RDM LUN's
The scan with timeout and the upgrade / installation will complete after few min's or hours depending upon number of RDM presented to the ESXi host
In the kernel you might see below
This happens every single time. I am yet to do a 6.5 upgrade successfully using any method. I would be interested to hear from anyone who has.
I have no RDMs mapped and it doesn't seem to matter whether I use VUM, offline bundle or booting from an ISO. I get the same result if upgrading from 5.5 or 6.0.
I have tried OEM & generic images. I make sure I remove all incompatible VIBs.
Every now and then I get one that I think has worked. It survives the first reboot but the instant you apply any updates - PSOD.
The "simple" process of upgrading 30 hosts from 5.5 to 6.5 has taken over a year. And the best part, 5.5 was perfectly stable and generated no support tickets. We only upgraded from 5.5 because VMware wouldn't support it anymore. Logging lots of tickets now !!