Booting the Raspberry Pi from a USB hard disk or pen drive.

There are a number of Raspberry Pi SD card images which are pretty big, some as much as 128GB such as Hyperpie.

So do you spend £40 on a SD card which might end up being 1MB too small or do you use that old 256GB external USB drive you have in the cupboard. You use the HDD of course.

There are two ways to get the Pi to boot from a USB mass storage device, one is via a one-time setting which afaik cannot be changed back, I won't go near that.

The other option is so simple it's obviously the correct choice for most usages.

You will need an SD card to start the Pi and point it to the USB drive. This can be small, slow and old, it really does not make any difference.

So here's what I did (in Windows).

1, Use Etcher in Dangerous Mode to write your image to the external HDD in the same way you would to an SD card.

2, Mount the drive and copy all files from the Boot partition to the root of the SD card.

3, On the SD card, edit the file 'cmdline.txt'.
  You will see a line with the text 'root=/dev/mmcblk0p2' replace this with 'root=/dev/sda2'

4, Plug in and enjoy.