We had a number of customers report that after upgrading their Apple Mac Pro 6,1 to the latest ESXi 6.7 Update 1 release, the system failed to boot and the following error was seen on the console:
Shutting down firmware services...
Mutiboot buffer is too small.
A few customers made the observation that this was in result of a newer version of the Apple Firmware that was included in the Mac Pro 6,1 systems which seemed to have cause this problem. In addition, this also impacts brand new installations of ESXi 6.7 Update 1 on new Mac Pro 6,1 that includes a newer version of the Firmware. At this time, is it NOT recommended to upgrade to ESXi 6.7 Update 1 if you are running the Mac Pro 6,1. A request has already been made to the VMware HCL team to remove Mac Pro 6,1 for ESXi 6.7 Update 1 and hopefully should be reflected by later this evening.
UPDATE 04/11/19 - This issue has been resolved in vSphere 6.7 Update 2
Note: This issue can also affect the Apple Mac Mini, which is not an officially supported platform for ESXi.
In the mean time, VMware has published KB 59660 which provides a workaround that may help customers who have upgraded their Mac Pro 6,1 to ESXi 6.7 Update 1 to rollback to the previously installed version of ESXi. Please refer to the KB for more details and you can subscribe to the article for future updates regarding this issue.