Error 15: Could not find file

After patching one of my ESX hosts, it became inaccessible after the reboot.  I DRAC'd into the server and found the following error:

Error 15: Could not find file

running the vdf -ha command showed that /boot was nearly full.  This host was upgraded from ESX 3.5 to 4.1 and was never "cleaned up"

Go into troubleshooting mode and run the following command to removed the old ESX 3.5 files:

cleanup-esx3 –f

The ESX host was then rebooted, and came back up without incident.