Do you have a good power supply for your RPi 3B, and maybe use an own power supply for your external SSD, because often the usb does not deliver enough current to power the SSD and allow it to boot directly on it.
Can you boot with SD card and check if the option in dietpi-configadvanced-options the line : USB boot support is set to ON ?
Is it an headless install ? perhaps you can connect a screen to find whats the issue is …
Your RPi 3b has 4 USB 2.0 ports, which are specified for 500mA per Port, which is only 2,5W on one port.
(In Fact the RPi 3b is limited to a total of 1200mA for all 4 ports. So you can get the 2,5W per port for only 2 devices, if you connect more it will get reduced, depending on the power consumption of the connected devices).
At first I also thought there wouldn’t be a problem connecting a SSD directly, without an extra power supply. But then I had random system freezes from time to time and the powered USB adapter was the solution to get rid of this.
What I’ve been doing is soldering a USB 3.0 boards power connections onto the 5v and ground GPIO pins in order to create a more secure jack (fuck micro USB). Based on what your saying even if I supply like 3A, the USB ports can not take advantage of the extra current?
It is as I said, the RPi3b uses USB 2.0 ports, which can deliver 500mA per Port, but overall only 1200mA for all 4 ports in total despite how much amps your power supply can deliver.
Jap RPi ports are not designed to power HDD/SSD. Especially on boot process, this could lead to issues. From our side we always recommend to use own PSU or powered USB hub.
With current firmware, it’s 1.2A overall on all USB ports by default on all RPi models. But even this is inefficient in many cases, also since power draw of the drive can peak much above of what vendors state during boot and I/O peaks. USB-only powered 2.5" drives (inclusind SSDs) are the #1 reason for failing USB boot and data loss.
As said, peak power draw can be obviously higher. And yes, the amount of power the RPi itself consumes during boot and peak situations, in combination with the used RPi PSU, can be another limiting aspect.
It’s hard to tell exactly if/why/when USB power isn’t sufficient, but our experience with a lot of such cases show it isn’t reliable. And besides, also RPi engineers tell the same. It might work in some cases, it also might have worked with one kernel version or system config, and then fails after a kernel upgrade or config change. Since it can lead to data loss, I’d simply not risk it .