Rob – many thanks for your comment. Yes, looks like I got it wrong. I’ll edit and re-post accordingly.

Your comment also spurred me to check out the man page. The -C (uppercase option) is interesting, as it can print a progress bar to the screen while the check is happening.

You can specify -C 0 (that’s a zero) to show this. The absence of the numeric argument caused the error you mentioned.

Best wishes.