unable to open '/dev/dm-1'
unable to open '/dev/sdc'
Generating locales (this might take a while)...
Generation complete.
pwconv: failed to change the mode of /etc/passwd- to 0600
Using CD-ROM mount point /cdrom/
Identifying... [6ff159b21d13d5addf7bcd3d3664ccb6-2]
Scanning disc for index files...
Found 2 package indexes, 0 source indexes, 0 translation indexes and 1 signatures
Found label 'Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)'
This disc is called: 
'Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)'
Copying package lists...gpgv: Signature made Wed Apr 20 22:29:18 2016 UTC using DSA key ID FBB75451
gpgv: Good signature from "Ubuntu CD Image Automatic Signing Key <cdimage@ubuntu.com>"

Reading Package Indexes... 0%

Reading Package Indexes... Done
Writing new source list
Source list entries for this disc are:
deb cdrom:[Ubuntu 16.04 LTS _Xenial Xerus_ - Release amd64 (20160420.1)]/ xenial main restricted
Repeat this process for the rest of the CDs in your set.