Make sure it is in the dual internal SD module page 70 [URL]  
We want to use vFlash as permanent first boot device. Is it possible? Right now I can't remove the "boot once" for vFlash.  All of our SAS disks are setup as part of vSAN. We just want to use a small 8G flash drive for ESXi OS.        
iDRAC: unable to use vFlash as first boot device every time