Error -1101 or -1102 with Macintosh HD

When Apple performs a system update, it will change the Macintosh HD in a way that makes Retrospect think the disk has changed. The result is an error:

Can't access volume Macintosh HD, error -1101 (File/Folder Not found)

If you try to browse the disk from Sources, you will probably see a -1102 error:

This happens because the Macintosh HD appears offline with the ~ symbol in the left margin.

1. To correct the problem, Go to Sources and select the instance of the Macintosh HD with the ~ next to the name and click Remove at the top of the window.
2. Go to Scripts and select your backup script.
3. Click on Sources and select remaining instance of the Macintosh HD with the circle next to it. Add a checkmark as shown below and click Save at the top.