no problem
BEFORE:
-I installed ESXi in the internal (32Gb) USB stick (I use an external bootable 8Gb USB stick with ESXi ISO injected with Realtek network driver)
-test that the ESXi system boot correctly (without a Datastore)
NOW:
1.boot GPARTED and create an unformatted partition in the free space of USB stick (I leave some free space between the last used partition and the new one, but i don't know if it is important)
2.using the terminal in GPARTED set the type of partition (if the new partition is /dev/sdb2):
#gdisk /dev/sdb
{press t}
{select partition 2}
{type fb00}
{press w}
{confirm with Y}
3.now change the partition number (from 2 to 10) to be sure that it will not be touch in future update
#sfdisk -d /dev/sdb > esxi2.txt
edit the text file and change
/dev/sdb2 : start= 13500416, size= 16775168, type=0FC63DAF-8483-4772-8E79-3D69D8477DE4, uuid=50D5FE33-CDC7-413B-AF11-C32BF433AA85
to
/dev/sdb10 : start= 13500416, size= 16775168, type=0FC63DAF-8483-4772-8E79-3D69D8477DE4, uuid=50D5FE33-CDC7-413B-AF11-C32BF433AA85
and move the line on the end of text file
Then write back the partition table
#sfdisk --force /dev/sdb < esxi2.txt
4.boot ESXi system (from modified USB stick).
From terminal (I use SSH):
check partition
#partedUtil getptbl /dev/disks/mpx.vmhba32:C0:T0:L0
and format it
#vmkfstools -C vmfs6 -S USB-Stick /dev/disks/mpx.vmhba32:C0:T0:L0:10
THAT's ALL
Now you see in DataStore a new DataStore "USB-Stick" and you can create folder, upload file, create VM, etc...
WITHOUT change "usbarbitrator"
SO you can attach USB HDD on external port to pass the USB DEVICE to VM