That's odd....
Did you patch & load the Rockbox bootloader before you installed Rockbox to the HDD?
Well first--Before you do anything, please back up your music.
I'm assuming you've installed Rockbox on your player before, but if I remember correctly, there's a step where you need a copy of the original iRiver firmware in a specific folder so the Rockbox Installer can patch and load it. Not all players make you do this step.
The only other thing I can think of is to plug the iRiver into your computer and run a diagnostic on the HDD (like chkdsk or something)--scan for bad sectors or other errors and see if you can fix them. Then reinstall Rockbox from scratch.
If that doesn't work, reformat the HDD, install the original iRiver firmware, then install the Rockbox bootloader and Rockbox.
I think Error -4 relates to the player being unable to read a block of memory, but I don't know if it refers to the HDD or the built-in memory.