This morning my Mac’s backup (time machine) reported that the backup disk (a sparse bundle on a time machine) was read only and it couldn’t backup.  A bit later I noticed the error was more serious: “The disk “Backup of Pra…” was not repairable by this computer.  It is being made available to you with limited funtionality.  You must backup your data and reformat the disk as soon as possible.” (sic).

So I fired up Disk Utility; hit open; browsed to find that sparse disk image.  Found it.  But, it wasn’t available to open;  grayed  out.  Browsing the web I found the suggestion that I should drag the image into Disk Utility’s window – that opened it.

At that point I tried letting disk utility try to repair the sparse image.  No luck “Invalid sibling link.”  Argh.  I found a posting that implies I can buy a more potent disk repair utility ($50), but instead I’m just going to discard the backups and start over. 

This, and reading various postings about the problems people are having with their Time Machine backups is quite distressing.