[CentOS] sha256sum a dvd

CentOS-6.9

I am trying to verify a locally created dvd.  I am using sha256sum in
this fashion:
sha256sum /dev/sr0

Which gave this result:

sha256sum: /dev/sr0: Input/output error


So I tried this:
sha256sum /dev/cdrom

Which, after some time, also produces:

sha256sum: /dev/cdrom: Input/output error

What does this mean and how do I fix it?


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