Before you upgrade an ESX host:

A.
back up /var/logs, /usr, /bin and the dmesg output

B.
back up /proc, /mnt, /tmp, custom scripts and all VMFS datastores

C.
back up /home, /etc/init.d and /boot

D.
back up /etc/passwd, /etc/groups, /etc/shadow, /etc/gshadow, custom scripts and local VMFS

Explanation:
vSphere Upgrade Guide ESX 4.0, ESXi 4.0, vCenter Server 4.0, vSphere Client 4.0, page ‘Before you upgrade an ESX host, back up the local VMFS file system. This ensures that you will not lose data during the upgrade.
Procedure
* Back up the files in the /etc/passwd, /etc/groups, /etc/shadow, and /etc/gshadow directories. The /etc/shadow and /etc/gshadow files might not be present on all installations.
* Back up any custom scripts.
* Back up your .vmx files.
* Back up local images, such as templates, exported virtual machines, and .iso files.’