Bootable USB drive on OSX

Movign a bootable ISO to USB drive on OSX:

- Insert the USB drive. For portability, format as FAT 
- The drive will mount, unmount the volume (but do not "eject")

    user $ sudo diskutil unmountDisk /dev/disk2
    Unmount of all volumes on disk2 was successful

- I need a GPT partition, in place of some junk here, so OSX formating tools will work just fine:
    
    user $ sudo diskutil partitionDisk /dev/disk2 1 MBR "Free Space" "%noformat%" 100%

- Finally, move the drive image over to the USB drive. 

Home Brew can provide a pv (Pipe View) utility that will nicely show where the 8gb transfer is at: 

    user$ dd if=windows7pro64.iso | pv | sudo dd of=/dev/disk3
     175MiB 0:05:28 [ 470KiB/s] [    <=>                                          ]

else, venerable dd will work. Native OSX version does not have the progress indicator. If DD 
received SIGINGO (kill -INFO $pid), it will display copied bytes.

    user $ sudo dd bs=1m if=Downloads/2017-09-07-raspbian-stretch-lite.img  of=/dev/r conv=sync

    user$ sudo kill -INFO 24088
    325+0 records in
    324+0 records out
    339738624 bytes transferred in 53.801800 secs (6314633 bytes/sec)

    user$ sudo kill -INFO 24088
    1768+1 records in
    1769+0 records out
    1854930944 bytes transferred in 290.152479 secs (6392952 bytes/sec)

Bytes are hard, a clonky helper is a function that will calculate how many megabytes are left:

function dd_remaining () { total=$1; shift; current=$1; shift; echo $(( ($total - $current)/1024/1024 ));}

    total=3319478272 dd_remaining $total 3174481920
    138