By default, Raspberry Pi boots up and stores all of its programs on a microSD memory card, which has rather limited bandwidth. On the Raspberry Pi 4, the memory card slot can achieve a theoretical maximum of 50 MBps, which is double the 25 MBps maximum on the Pi 3B+ (and other 3 series). you get more speed if you attach an external drive to one of the Raspberry Pi’s USB ports, especially with the Pi 4 offering USB 3.0 connections that have a theoretical maximum of 660 MBps.
Unfortunately, at this time, you can’t boot a Raspberry Pi 4 off an external drive. But you can force the Raspbian OS to use an external drive for its “root” partition, which holds all of its programs and data. So, in effect, you have a small boot partition on a microSD card but the meat of the entire operating system would still live on your speedy SSD or Flash Drive. A future firmware update will allow the Pi 4 to boot off of external drives, no microSD card required, The instructions directly below work on a current-day Pi 4 or an earlier model, but if you want to boot your Pi 3 off of an external drive scroll down to the next section of this page.Read More