I don't know retropie at all, but it's a Linux system, which I do have knowledge about.
According to
this article retropie mounts external drives unter
/media like most of today's Linux distributions. As described in the article, you can see the path to the mounted drive via the command "df -h" or simply "mount". Then, you can dive deeper into the folders with the commands "ls -l" (
li
st the contents of the current folder) and "cd <foldername>" (go into the specified folder) until you know the whole path to attract mode's files (e.g.
/media/usb0/mame).
Maybe retropie's terminal console even has
tab completion to ease the typing. Then, you'd only have to type some first letters of a command or argument (e.g. the folder's name) and the tab key will complete them. Double-tabbing will show you all possible folders for the typed characters so far. Using this method, a simple "cd /media" (without pressing the Enter key!) followed by (double) tabbing and giving it some first letters of the next folder(s) could show you the whole path at the end.
I hope I could make myself somewhat clear.
If that doesn't help, you may have to wait until someone with more retropie knowledge shows up.
edit: On the second thought, an easier way could be to learn the usb path under
/media (like /media/usb0) as described above and then just add the rest of the path
on the drive you can look up on
any other system. Sometimes, I think too complicated.
