It can work (only with Samsung XP941 drive at this point). After much googling of posts and use of files written by smart and generous people I managed to fumble my way through it all. In case it's of help to anyone else this is what I did to get ESXi5.5 running off a USB key plugged into the Asrock Z97 extreme6 mobo with M.2 SSD running. It's highly likely there are better ways to go about it, but this is what worked for me.
1 - Don't download ESXi - it does it automatically later, when the install package is customised
2 - Install VMWares Image Builder PowerCLI and Prerequisite Software (took a lot of installs) See:
vSphere Documentation Center
https://my.vmware.com/web/vmware/searchresults#site=my_download&client=my_download&getfields=*&outpu...
3 - Run Windows Powershell AS ADMINISTRATOR and set execution env to RemoteSigned
http://security.stackexchange.com/questions/1801/how-is-powershells-remotesigned-execution-policy-di...
https://www.cogmotive.com/blog/powershell/allowing-powershell-scripts-to-execute
NB - make sure you set the execution policy in the same verion of PowerShell as PowerCLI - ie 32 or 64 bit, they are different
4 - Download 2x update packages for Intel network socket plus Samsung M.2 drive and chipset SATA (e-10000 and sata/ahci) http://vibsdepot.v-front.de/wiki/index.php/List_of_currently_available_ESXi_packages
5 - Install ESXi customeriser - http://www.v-front.de/p/esxi-customizer-ps.html#download
6 - Run step f - http://www.v-front.de/p/esxi-customizer-ps.html
This DOWNLOADS the ESXi install AND adds the zip files to it
I ran: .\ESXi-Customizer-PS-v2.2.ps1 -v55 -obDir C:\temp\z97\zip (with the 2 zip files for ahci and e10000e in the directory)
7 - BIOS - set primary graphics = PCIe (and some card in place)
8 - INSTALL TO USB key - I had to set an option formatwithmbr right at start of ESXi install by pressing Shift-O (not zero)
NB also note put a "SPACE" in front of formatwithmbr https://communities.vmware.com/message/1831114#1831114
Means 2TB drives max, but I failed finding UEFI combination to allow GPT install even tho it should be supported.
