I stumbled over this once, since it doesn't seem to be documented anywhere in the official documents, not in the Knowledge base and not in the vSphere 5 Guides.
But if you run the installation it asks you to "select a disk that has a capacity greater than 910 MiB".
So that would be the answer I guess
That would make sense all right . I have found some people in other forums saying they were running ESXi from 1 GB keys.
I've done mine on a 1 GB USB stick and working fine!
I'd recommend at least a USB 2.0 device to avoid long boot times, or USB 3.0 if you can find it.VCDX #104 (DCV, NV) ஃ WahlNetwork.com ஃ @ChrisWahl ஃ Author, Networking for VMware Administrators
From the Documentation
Storage Requirements for ESXi 5.0 Installation Installing ESXi 5.0 requires a boot device that is a minimum of 1GB in size. When booting from a local disk or SAN/iSCSI LUN, a 5.2GB disk is required to allow for the creation of the VMFS volume and a 4GB scratch partition on the boot device. If a smaller disk or LUN is used, the installer will attempt to allocate a scratch region on a separate local disk. If a local disk cannot be found the scratch partition, scratch, will be located on the ESXi host ramdisk, linked to /tmp/scratch. You can reconfigure scratch to use a separate disk or LUN. For best performance and memory optimization, VMware recommends that you do not leave scratch on the ESXi host ramdisk. To reconfigure /scratch, see Set the Scratch Partition from the vSphere Client. Due to the I/O sensitivity of USB and SD devices the installer does not create a scratch partition on these devices. As such, there is no tangible benefit to using large USB/SD devices as ESXi uses only the first 1GB. When installing on USB or SD devices, the installer attempts to allocate a scratch region on an available local disk or datastore. If no local disk or datastore is found, /scratch is placed on the ramdisk. You should reconfigure /scratch to use a persistent datastore following the installation.
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