I think this information will be a great help for you: http://www.vladan.fr/plug-in-the-external-usb-drive-to-esx-server-directly-to-backup-or-copy/
iSCSI SAN software
why would you want to backup ESXi itself?
VMware Communities User Moderator | VCP | VCDX
Now available: Paper - vSphere 4.0 Quick Start Guide (via amazon.com) | PDF (via lulu.com)
Here's a script that does exactly that if you wanted to backup your ESXi configurations - ghettoHostBackupManagement.pl
VMware vExpert 2009
VMware ESX/ESXi scripts and resources at: http://engineering.ucsb.edu/~duonglt/vmware/
If you find this information useful, please award points for "correct" or "helpful".