Creating a bootable USB with diskutil and dd

Question

I am trying to create a bootable usb drive using diskutil and dd. I have had some success but not exactly what I wanted or expected. I am creating a bootable usb for MemTest86. I have an img file provided by PassMark. I first partitioned the drive using:
diskutil partitionDisk /dev/disk4 1 GPT "HFS+" "MemTest86" 100%
output:
TYPE NAME               
SIZE  
IDENTIFIER
   0: 
GUID_partition_scheme                   
*3.9 GB
disk3
   1:                   
EFI EFI                
209.7 MB  
disk3s1
   2:             
Apple_HFS MemTest86          
3.6 GB
disk3s2
This gives me what I would expect, the first partition is an EFI partition of size 209.7MB and the remaining 3.6GB, on a 4GB drive, being allocated to the second partition, /dev/disk3s2, MemTest86.
To copy the img to the disk i first unmount the partitions on disk3
diskutil unmountdisk /dev/disk3
I then run dd
sudo dd if='//Volumes/Mac Mini Backup/Archives/MemTest86 Pro/memtest86-pro-usb/memtest86-pro-usb.img' of=/dev/disk3 bs=1m
Now, the file does end up on the usb drive but..... The data is written to the first partition which used to be the EFI partition. It is now named Untitled and it is a FAT16 partition.
diskutil list /dev/disk3
output:
TYPE NAME               
SIZE  
IDENTIFIER
   0: 
GUID_partition_scheme                   
*3.9 GB
disk3
   1:  
Microsoft Basic Data                    
51.4 MB
disk3s1
   2:                   
EFI                    
103.8 MB  
disk3s2
Now, although, this isn't what I expected it does sort of work. When I boot the system into the Startup Manager there are 2 extra drives to boot from, both named "efi boot", all lower case. Both boot into the MemTest86 utility which is my goal but I first didn't expect the partition layout that I got and I can't figure out why I get 2 boot icons to select from. I am guessing that the format of the img file is causing the partition to be a FAT16 partition but I am waiting to hear back from PassMark regarding that.
So if anyone cares to shed a little light on what is happening I would appreciate it.

Answer 1

The only known way to make it work on an external drive is by first installing Windows onto an internal drive, then cloning the install to an external Thunderbolt drive. Thunderbolt is seen as an extension of the internal bus, so Windows doesn't see it as an external device.