Permissions issue when copying files over SMB to server on OS X 10.9 Mavericks
Usually the authentication prompt is in relation to the destination for your copy. 

Thus, from what you're describing, you should enter your Gentoo login credentials. However, I suspect the owner/write permission are too restrictive on that specific volume. Have you checked owner user/group on the problematic volume vs. the other "problem-free" volumes? Are the read/write permissions the same?

To change the permissions without resorting to Terminal, highlight the folder you want to put the user file in and select Get Info (Cmd+i). At the bottom are the permissions. Make sure that where it says "everyone" that it also says "Read &amp; Write." You may have to click the little gold lock at the bottom of the Get Info pane to adjust this. Once you do this, you will be able to copy your folder over.