If you select the Time Machine menu item, it will give you a more complete explanation. Normally though it just means that your disk has been made inaccessible for some reason; either it's not connected, or if it's on the network, it has become disconnected from your network (or your computer has). Try forcing another backup and you should be informed.
-- You received this message because you are subscribed to the Google Groups "MacVisionaries" group. To unsubscribe from this group and stop receiving emails from it, send an email to macvisionaries+unsubscr...@googlegroups.com. To post to this group, send email to macvisionaries@googlegroups.com. Visit this group at http://groups.google.com/group/macvisionaries. For more options, visit https://groups.google.com/d/optout.