I had a similar problem when I was trying to send books to the internal memory so that I could have collections.. Kovid said that the error code indicated a missing directory.
I was told to try the following commands
Code:
prs500 mkdir /Data/media
prs500 mkdir /Data/media/books
and if I remember correctly I got an error for the first one saying the directory already existed, but the second one worked. Ever since then I have been able to send books to the internal memory without problems.
Other options such as ls within the prs500 command allow you to list existing files and folders.
The commands need to be run from a command/terminal window. Note that Calibre must not be running when you try this as it would already ahve grabbed the reader