Please check this link :
Guys there are trying to do same as you with a USB boot without SD card.
Guys there are trying to do same as you with a USB boot without SD card.
That is all above my skill level. So I guess I wait. I was reading that 20.10 is ready to boot from SSD.
Do it man 
there are steps that is easy to understand, with ready to use command lines.
Just search inside the page for :
I had a similar problem after following the instructions
and copy paste commands he gave. Great opportunity to learn and get new skills
Did that followed all commands and now my SD will not boot. I am starting again. I used my backup SD and I am back with all the fixes and mods. Just no SSD boot.
soma72 on Wimpy World gave me the answer. It had nothing to do with commands, It turns out that there is a bug in the OS that prevents booting from USB 3.0 without modification to the boot menu. I am able to boot without any modification using USB 2.0. I hope this is on the agenda to be patched.
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I have the same problem. I use WD green 120 GBSSD for Ubuntu Mate 20.10 on Raspberry Pi 4 4GB and it doesnt boot wnen connected via USB 3. But is boots on USB 2.
However, I have seen Leepspvideo on you tube and he doesnt mention any issue while booting via SSD. Could it be something to do with SSD?
Raspberry Pi OS required that I set up VendorID and DeviceID quirk, for the HDD to be recognised as a boot device.
I had a problem booting with a USB / mSata adapter using a Jmicron controller. Now I only take AsMedia controllers
I have tried two different adapters and two different SSDs this is a bug that is fixable. You should report it as a bug as I have done. It won't be important if I am the only one reporting it.
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How do I find out how to get the vendor ID etc? And of course how to modify the OS for this? Thanks.
@Sparks I really don't know the code for Ubuntu 
. But assuming it is same as Raspberry Pi OS(both being Debian based) , check this out. However, if you are not sure of the code, I suggest that an expert from this community advises us.
https://www.raspberrypi.org/forums/viewtopic.php?f=28&t=245931
@Sparks yesterday I was being lazy when I posted that workaround idea. Didnt know whether it would work.
Today I tried it. AND ...IT...WORKED...LIKE...A...CHARM!!
You will need to be able to edit the /boot/cmdline.txt file.
Login to Raspberry Pi OS on a sd card and from there you should be able to use the quirk
I have an update! I purchased a Silicon Power M.2 SSD and a StarTech external case and it booted from USB 3.0. I am elated.
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you should be able to use your other ssd too
check the method I posted above
OK I did the commands over, but not sure where I went wrong. Please look at my screenshots. I am thinking that the tutorial is for just getting the best speed for the SSD, not for booting the SSD.
Sorry for the delayed reply. As I mentioned, I am not an expert at this. But we can take it step by step.
I assume you are trying to boot through the old SSD, not the Silicon Power M.2.
So do not connect that one.
Secondly, load Raspberry Pi OS on a Micro SD card and boot from there.
After that carry out the procedure as mentioned.
I also do not see any other text in the cmdline.txt. I think There was some text in the file which I edited.
Make sure that you are editing the cmdline.txt which is present on the SSD.
I would suggest to reload the SSD with fresh copy of Ubuntu Mate before proceeding with any of the above. If that doesnt work, I really am sorry I don't have any other suggestion. It just worked for me.
OK I did that my mistake was I did it using Mate not Raspberry Pi OS. That worked, thanks.
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Glad to hear. I never thought that such cross platform tricks would work.
Now I am waiting to hear from LoveRPI as my Pi 4 B HDMI has failed. I only bought it a month ago. Oh well I am annoyed.
Good information thanks for sharing
vmware
Didn't help the RPi 3 USB boot problem with the current Mate image - 20.04.1/20.10. The older 18.04.2-beta1 works fine.