My Book World Edition no longer Backing Up?

I have a My Book World Edition in my network.  My main PC runs W7, my other PC XP.   I installed the WD software on both PCs and it then backed up both PCs successfully.  However, since then I don’t think it has backed up any new files or responded to new versions of the files. 

A possible clue to the problem is the WD Anywhere Backup screen.  For my main PC (W7) it indicates My WD_Backup "Pending (Destination not Available).  The other PC (XP) WD_Backup_1 it appears to be backing files up, but I see no change when I search the My Book World Edition Y folder, where the backups are held.

I am 100% up to date with Windows support downloads etc.

Suggestions appreciated.