Under 4.1, USB is no longer listed in the documentation as a bootable device, though boot from SAN devices via HBAs is now supported.

I’ve not checked whether you can actually install ESXi Installable 4.1 to a USB drive. It may well be possible, but I suspect it’s dropped off the “Supported” list of options.

The only reason I can see to drop support for putting Installable onto USB is to encourage people to purchase and use ESXi Embedded from their hardware supplier instead.

**UPDATE 2nd March 2011**

VMware have posted a clarification Knowledgebase article about the support for USB and SD for booting ESXi.

You can install ESXi 4.x to a USB or SD flash storage device directly attached to the server. This option is intended to allow you to gain experience with deploying a virtualized server without relying on traditional hard disks. However, VMware supports this option only under these conditions:

• The server on which you want to install ESXi 4.x is on the ESXi 4.x Hardware Compatibility Guide.

  AND

• You have purchased a server with ESXi 4.x Embedded on the server from a certified vendor.

  OR

• You have used a USB or SD flash device that is approved by the server vendor for the particular server model on which you want to install ESXi 4.x on a USB or SD flash storage device.

If you intend to install ESXi 4.x on a USB or SD flash storage device while ensuring VMware support for it and you have not purchased a server with embedded ESXi 4.x, consult your server vendor for the appropriate choice of a USB or SD flash storage device.

So as I suspected, it’s only supported if you install either using “Embedded” or on a device approved by the server vendor.

It does work absolutely fine on a normal USB stick, the host that this web server runs from boots from such an item in fact. It’s just not supported by VMware.

Worth noting that an approved 2GB USB stick from HP will cost you approx £75, about 15 times the going rate…