linux – Can not modify the permissions of the Raspbian folder?

I have a USB key mounted on a folder in my raspberry pi. I'm using this drive to host an FTP server with Gadmin-ProFTPD. However, I can not download anything, regardless of the permissions granted to a user. It looks like this:

ftp> put test.txt
local: remote test.txt: test.txt
200 PORT successful command
550 test.txt: permission denied
ftp> 

I think this has to do with the file permissions because flash drive folders have access control Change the content permission defined at Single owner. However, when I change it to say No matter who and press OK, the next time I open the properties, it's back to Sole owner. This happens regardless of my response to Do you want to recursively apply these changes to all files and subfolders? (what I do). Why can not I change permissions this way and how can I overcome this problem?