Thanks to David Passell for the following important info.
"Time machine is a very good program for going back if something goes wrong, but don’t use it as a backup for things you might want to go back to in the present. If you open Time Machine preferences you will note the last line, 'The oldest backups are deleted when your disk gets full.'
"The nasty thing about it is that you get the warning after it has happened. (A clue is when menu icon turns for a long time). Then you open Time Machine and find out nothing but the last month or less is still backed up. If you think you can go back to that choice picture/article/etc. you deleted to save space, you are out of luck.
"I have had it happen to me more than once. The following conditions may cause this to happen:
• Forgetting to exclude Parallels virtual machines.
• Connecting a new disk drive and putting a clone of your system or a sandbox on it.
• Creating or downloading a large video or program file (Lion upgraders take notice).
It is probably a good idea to turn off Time Machine before doing a large download or any operation that may cause a backup of it all to occur. Before starting Time Machine again go to Options and exclude the new drives."
At this point David offers his closing comment, "One last thing about Time Machine: I am considering purchase of a larger drive to replace the 500 GB drive I have been using since 2008. (I lost those early time machine records long ago). However, I have seen no instruction on transferring an existing Time Machine to a new drive to preserve the old ones and continue with new. Could I clone the Time Machine to a partition in the new drive, and then select the partition in the new drive as 'Use for Time Machine'?"