Still can't figure out the permissions problem for copying to second disk

[Tyrannocaster]

I have now switched to Thunar, eliminating that whole issue. It works better, but honestly, copying files is still wildly erratic. Sometimes I can and sometimes I can't. For example, I just stuck a flash drive in, it mounted, and I tried to copy a file from my second drive to it using an instance of Thunar that was already open. Wouldn't do it. However, I COULD copy it to the flash drive IF I copied it to the instance of Thunar that I opened by double clicking on the flash drive's icon.

I can't believe this is the way this is supposed to work. Can someone explain what the rules are for this?