There is very little information available on the Internet regarding troubleshooting music sync errors in Rhythmbox. I was getting a "could not open resource for writing" error when trying to sync music. I finally tracked down the issue. One issue, anyway, I'm sure this can happen for myriad reasons. In this case the same file existed in 2 locations within my music folder structure. When syncing to the phone the file names/directories are normalized. When the 2nd file was attempting to sync, the file already existed on the phone and the sync process would crash. By removing the duplicate file from the computer the sync process continued successfully.
I'm sticking this next part here so I don't have to go digging around the Internet again in the future.
Unrelated to the file crash, but related to syncing music via Rhythmbox: to sync to an abitrary device (in my case, an SD card), place a file called ".is_audio_player" in the root directory of the device. In that file I placed this text:
audio_folders=music/ playlist_path=playlists/ folder_depth=2 output_formats=audio/mpeg,audio/mp4,audio/aac,application/ogg,audio/x-ms-wma
Then Rhythmbox will detect it as a "sync-able" location. You may have to close/re-open Rhythmbox and possibly umount/remount the device.