SSD firmware upgrade requires iso on a pendrive: how to make that bootable?
First find USB key disk number from diskutil list.  This method will destroy everything on the disk you copy to so take care. Mine was /dev/disk3 and looked like this:  

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *32.0 GB    disk3
   1:                        EFI EFI                     209.7 MB   disk3s1
   2:       Microsoft Basic Data FAT32                   31.8 GB    disk3s2


Make sure no volumes are mounted, then use dd to copy the unmounted ISO file to your device (do not append a partition number, so do not use something like /dev/disk3s2).

This worked for me with the NVMe SSD-970 EVO Firmware ISO 2B2QEXE7 from here.

[hali@macOS] ~ $sudo dd bs=4m if=/Users/hali/Downloads/Samsung_SSD_970_EVO_2B2QEXE7.iso of=/dev/disk3
Password:
4+1 records in
4+1 records out
17825792 bytes transferred in 8.546376 secs (2085772 bytes/sec)
[hali@macOS] ~ $sync
[hali@macOS] ~ $


diskutil list now looks like this:

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:                            Samsung                *32.0 GB    disk3


and in Disk Utility like this:



Reboot holding &#8997; and pick the yellow boot option on USB key called "Windows".  It should load the firmware utility fine (it did for me on 2015 MBP).



Afterwards, as mentioned in Arch Wiki the ISO signature needs deleting. 


  Warning: This will irrevocably destroy all data on /dev/sdx. To restore the USB drive as an empty, usable storage device after using the Arch ISO image, the ISO 9660 filesystem signature needs to be removed by running wipefs --all /dev/sdx as root, before repartitioning and reformatting the USB drive.  


As wipefs doesn't exist in macOS, this can be achieved by using gdisk to write a new gpt partition table.  Simply reformatting in Disk Utility didn't work to recover use of the USB resulting in lots of "Operation not permitted" errors.