Starting an ESXi installation that is a part of a vSAN cluster from a flash device imposes certain restrictions.
When you boot a vSAN host from a USB/SD device, you must use a high-quality USB or SD flash drive of 4 GB or larger.
When you boot a vSAN host from a SATADOM device, you must use single-level cell (SLC) device. The size of the boot device must be at least 16 GB.
During installation, the ESXi installer creates a coredump partition on the boot device. The default size of the coredump partition satisfies most installation requirements. (You need to configure the core dump partition)
If the memory of the ESXi host has 512 GB of memory or less, you can boot the host from a USB, SD, or SATADOM device.
If the memory of the ESXi host has more than 512 GB, consider the following guidelines.
You can boot the host from a SATADOM or disk device with a size of at least 16 GB. When you use a SATADOM device, use a single-level cell (SLC) device.
If you are using vSAN 6.5 or later, you must resize the coredump partition on ESXi hosts to boot from USB/SD devices. For more information, see the VMware knowledge base article at http://kb.vmware.com/kb/2147881.
Hosts that boot from a disk have a local VMFS. If you have a disk with VMFS that runs VMs, you must separate the disk for an ESXi boot that is not for vSAN. In this case you need separate controllers.