Although I find the idea of creating a tar straight onto the /dev/sdb1 partition a bit frightening, I can confirm that the reported issue itself is present for situations like "tar rf /tmp/test.tar /tmp/Sc <TAB> <TAB>" as well. Bash completion for "tar cf /tmp/test.tar /tmp/Sc <TAB> <TAB>" works fine.
Although I find the idea of creating a tar straight onto the /dev/sdb1 partition a bit frightening, I can confirm that the reported issue itself is present for situations like "tar rf /tmp/test.tar /tmp/Sc <TAB> <TAB>" as well. Bash completion for "tar cf /tmp/test.tar /tmp/Sc <TAB> <TAB>" works fine.