I wasn't able to find the answer to this posted anywhere online but I managed to resolve it on my own so I thought I'd post it here in case anyone else needed the help.
Phone: XT912 running CM10.2 with Safestrap 3.65
Problem: Data & System Backup Failed. "Error tarring split files"
Description: Backup wouldn't work no matter what I tried to do despite multiple successful backups earlier. Switching backup locations, clearing previous backups, NOTHING.
Solution: Realized this time I had rebooted directly into recovery rather than selecting Menu from Safestrap boot screen. SUCCESS.
Not sure what the issue was but maybe this will be helpful to someone.