Well, congratulations -- you discovered the real problem-- the old disk /Users and new disk /Users contents are not owned by the same userid (am assuming there is only one in each of the /Users tree?) -- so the files/folders are owned by different users (502 on old and 503 on new).
So am guessing you installed a new user a couple of times on new ssd and removed the old ones and on the old disk when it was the boot disk that you did that only once? Normally the first user you create is userid=501 and then if you use the Setup Assistant to migrate your old user home folder and contents to the new install, it doesn't create any new users it just copies the old 501 owned stuff from the old disk to the new disk and everything works as it did before.
Let's call your new username on the ssd disk yyy=503 and your old username on usb disk xxx=502 -- To solve the problem now is a bit more complicated as you need to find out if ownership of all the files in the old disk /Volumes/Old-USB/Users/xxx folder (where xxx is your username:group) is the same userid and groupid. To do that we need to do a little work to see how many different userid's own files in your xxx user, so try these commands:
cd /Volumes/Old-USB/Users/xxx
sudo find -x . -uid 502 | wc
sudo find -x . -ls | wc
You will be prompted for admin password so give it and then hit enter/return key.
These commands first set the default directory to /Volumes/Old-USB/Users/xxx and then look at all the files/folders below this to see how many are owned by 502 user and then see how many in total there are -- if those numbers are the same, then 502 user owns all files/folders which is what we're hoping. If they are not the same then tell us the numbers and we'll need more work.
If they are the same, then we need to make sure all are also owned by the same group (20 in your case), so to do that:
cd /Volumes/Old-USB/Users/xxx
sudo find -x . -gid 20 | wc
sudo find -x . -ls | wc
Again we're hoping for the same numbers, which means all files/folders are owned by group 20.
If both comparisons give the same numbers, then the easiest thing to do would be to create a new user when booted on the SSD disk and give it the userid code of 502 instead of accepting the default which is always one greater than the current highest, which in your case would be 504 (one greater than the existing 503 user). So run the System Preferences Users & Groups pane and create the new 502 user (and call it xxx just so you can have same username as you used to (perhaps you might want a different username so you can tell old from new??? -- and also make it an admin type user), then still logged in as the 503 user do the following (the following assumes the new short-form username you created is xxx):
cd /Volumes/New-SSD/Users
sudo mv xxx newxxx
sudo mkdir xxx
sudo chown 502:20 xxx
cd xxx
sudo cp -r -p /Volumes/Old-USB/Users/xxx .
This will copy all files from the old-usb/xxx user folder tree to the folder named xxx we just created in /Volumes/New-SSD/Users/xxx and maintain all the same permissions and attributes and hopefully all your problems are now solved. Since you didn't make any changes to the /Volumes/Old-USB disk, you can still boot off that and things should still work fine since we didn't really change anything there.
Does all this make sense? If not, ask questions first, not afterwards, because mistakes will cause problems - possibly big ones.
You're the one doing all this stuff, and if you follow instructions and I didn't make any typos and you don't make any mistakes things should be OK, but because I'm not there looking over your shoulder if there is a problem, this is all up to you -- no guarantees and no hard feelings if things don't work out, OK? The worst thing that might happen is you have to reinstall the OS and start over, but since we didn't change anything on the old disk, you will be able to try again if things don't work out correctly.
Good luck...
