tar compare or diff command line

Question

What is wrong with this?

tar --diff --gunzip --file=/media/disk/filename.tgz

This is GNU tar 1.23 on Ubuntu. The complaint is gzip:stdin:input/output error.

The following just hangs and there is no apparent disk seek activity.

tar --diff fz /media/disk/filename.tgz
tar --compare fz /media/disk/filename.tgz
tar -d vfz /media/disk/filename.tgz

Answer 1

tar --compare --verbose -z --file=filename.tgz ./directory > logfile

This or some variant worked. The point of redirecting to the logfile is to reduce clutter: the 'file not found' sort of differences stay on the console, the rest go into the logfile. I do not know why the other attempts by the OP did not work.