I have tried to copy files to a USB in both OS ( Operating Systems ) but i get the error:
The file operation was completed with errors.
Read-only file system.
Using user pi this should work, and does work on my system (Raspbian and Polaris, out of the box without changing any privileges). I don't know whether it works for other users.
Strange that it works well on my laptop ( not with the pi-top) with the same USB stick. Is it a issue that the pi user is not the root ?
I have written a description of what can be done to solve USB stick problems (applies to pi-topOS and Raspbian Stretch). See
Scroll to the bottom and use the link there.