I just ran into the same problem. The Time Machine said the backup was delayed because the external HD was not available. The situation persisted as I power cycled the MacBook Pro and WD My Book Studio (I'm using the FireWire 800 port).
But the system at the same time "saw" the HD and allowed me to enter into Time Machine and retrieve a previous document. So I retrieved a document from 2 months ago and restored it over the current version (same content so nothing was lost).
That action either triggered the Time Machine to notice the HD and complete the backup, or diverted my attention for long enough for Time Machine to do the backup.
So by restoring an old document via Time Machine, it seemed to fix the problem.
I have the same issue and its bugging me for month now. Everytime a backup completes, the next auto-backup fails and the "disk is not available" (its a NAS server), although it is still mounted and TM can see it in the disk in the list of available drives.
Each time i have to reselect manually to restart. I also tried to do a full reset (deleting preferences in the Library foldeR), same problem.
Has anyone found a fix for this? Hard reset fails, deleting the backup and recreating a fresh one, same problems. I'm running out of options!