Just a heads up for anyone else doing this - its slow or I did something wrong (Linux moron here ). I finally got round to reformatting the 6TB USB HDD I use on Kodi and adding the code to check it on boot:
Code: /storage/.config/system.d/[email protected]/fsck.conf
The only problem was the time taken to copy the c4TB of data from disk 1 to disk 2. It took over 26 hours. Started off really well telling me it would take c6 hours but it got slower and slower ending up at 40MB/s. I had done transfers using the exact same equipment, same data under Windows (NTFS -> ExFAT and NTFS -> NTFS) and that took c10 hours.
Its done now so I should no longer need to plug into a Windows PC and run chkdsk. Transfer speeds using SAMBA and FileZilla over my LAN look about the same as when the HDD was NTFS.