You'll need to find out the complete path to 'myusername'. That's probably something like '/media/SOME-HEX-CODE/myusername'. Then open a terminal and type
sudo chmod a+rx -R /media/SOME-HEX-CODE/myusername
I couldn't get that to work. Got an error.
Cannot find path a+rx
Ubuntu did not like the a+rx
So tried below, which worked
sudo chmod u=rwx,g=rwx,o=rwx /media/SOME-HEX-CODE/myusername
And then had to do that for each sub folder.
But thanks for the info as it got me in the right direction