So you might have seen that your disk utility reports ILLEGAL NAME in red and cannot fix it. Usual mac enthusiastic tip is to do reinstall. O behold, here’s how to fix it easily.
1) Boot to single user mode by pressing command+S at reboot until you seen black screen
2) when white cursors block appears after lots of text, type “fsck -f”
Wait until it does the repairs. DO NOT be alarmed if your computer fans start howling like vacuum cleaner. The repair process could take even 10-30mins at older macs.
3) once the white block returns or successful completion announcement comes, press enter (if there is no white cursor block”)
4) type “reboot”