Up til now, to boot a raspberry pi from SSD, you had to still boot from the slow SD card, with a small hack in the /boot directory. Now there are instructions to boot directly from the SSD. 

Doing this today to my RPi 4 / 4G with 500Mb SSD for my assistant (eventually talking glowing skull

Source: Raspberry Pi 4 Boot From USB – tynick.com | AWS, Linux, Raspberry Pi, and Home Automation

My configuration is a little bit different from his, I have Seeed’s X825 and X735 power management and SATA / USB drive adapter, all stuffed into a case. The tricky part of this was that to mount the hard drive on another system to flash it, you have to take it all apart to get the drive. The directions are not completely explicit, so I’ll spell it out:

If you have to take the whole thing apart, flash the SD and the SSD at the same time, put it all back together except for the part where you connect the USB ports between the drive and the Pi, Flash the EEprom as directed so it’ll change the boot order to SD > SSD . 

So when you boot from the SD card, it’s painfully slow if yo;u’ve gotten used to running your Pi from a normal recycled  laptop hard drive or even better a SSD. 

 

When you do the rpi-update command you get this scary-ish message:

pi@raspberrypi:~ $ sudo rpi-update 
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
*** Performing self-update
*** Relaunching after update
*** Raspberry Pi firmware updater by Hexxeh, enhanced by AndrewS and Dom
*** We're running for the first time
*** Backing up files (this will take a few minutes)
*** Backing up firmware
*** Backing up modules 4.19.97-v7l+
#############################################################
WARNING: This update bumps to rpi-5.4.y linux tree
See: https://www.raspberrypi.org/forums/viewtopic.php?f=29&t=269769

'rpi-update' should only be used if there is a specific
reason to do so - for example, a request by a Raspberry Pi
engineer or if you want to help the testing effort
and are comfortable with restoring if there are regressions.

DO NOT use 'rpi-update' as part of a regular update process.

########################################################
Would you like to proceed? (y/N)
 *** Downloading specific firmware revision (this will take a few minutes)
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   168  100   168    0     0   1024      0 --:--:-- --:--:-- --:--:--  1030
100  115M  100  115M    0     0   419k      0  0:04:41  0:04:41 --:--:--  607k
 *** Updating firmware
 *** Updating kernel modules


this follows with a long ish download and update