Time machine backup keeps running, total rising

Question

Hi all,
My Macbook Pro (late 11, osx 10.8.2, everything updated) has suddenly started having a roblem with time machine.  I'm using an external harddisk (Western digital Passport, 500 Gb) to backup my 250 GB Internal SSD.  It's been working fine for the 5 months I've had them both.  Now it has suddenly developed a problem where the Time Machine backup looks like it will go on for an infinite amount of time:
When the backup starts, after teh "Preparing backup..." cycle, it starts off displaying a sensible amount of backup to run (e.g., "backing up 0 MB of 980 MB").  Then after hitting or getting close to that total, the target amount rises contantly to always be ahead of the backup amount, i.e., it will start:
backing up 1.2 GB of 1.7 GB
backing up 2.3 GB of 2.5 GB
backing up 4.6 GB of 4.8 GB
etc
The longest I've left this for is about 25 minutes and it has no signs of stopping.  Is there a solution to this problem?  I noticed some similar questions on the forum but from 2 years ago, and some links that were suggested as possible answers are now no longer working.  Any help gratefully received!

Answer 1

I did find a solution which appears to be working, at least in my case.  Here is what I discovered was happening on my Mac Pro:
On one of my internal hard drives, I had created 4 different folders, and those folders contained various sub-folders.  Just for the sake of this discussion, We'll call them folders 1-4.  When I checked Get Info on the HD as a whole, it told me 1.12  of 2.0 TB was used, which was accurate.  When I checked folders 1-4, their totals all added up to 1.12 TB used.  However, When I went into certain SUB-folders within folders 1-4, and added up their totals using the Get Info command, it did not equal the total  HD space used when I checked the main folder as a whole. 
In other words, if I had 3 sub-folders in the "Main" folder #2 - and folder #2, using Get Info, said it contained a total of 450 GB, when I add up the 3 sub-folders contained within folder #2, it would equal 750 GB. 
These extra GB, located in only a couple of places on sub-folders of the HD, were confusing Time Machine.  It would act as stated above... it would take the total of 1.12 TB for that drive, attempt to back it up, but when it reached that total, suddenly, OS X ML would realize there was more data in folder #2's sub-folder, and the numbers would continually rise until the external backup drive was full, thus creating an error on Time Machine's backup.
Now, I don't know why this happened.  Apparently when I deleted certain files and emptied the trash, it didn't completely erase them.  I've tried using "Secure Empty Trash," but with the large video files I use, the empty procedure freezes before it can finish.  I cannot explain this phenomenon because I'm not well-versed on the inner-workings of OS X.
My solution was simple, albeit a pain.  I went into the sub-folder, selected all the items inside (folders, documents, videos, audio, etc.) and moved them out of that sub-folder.  Now, they are all floating freely in folder #2.  Then, I deleted that sub-folder, emptied the trash, and then re-created that sub-folder with the same name.  Finally, I moved all the items from that sub-folder back into the newly created one, and voila!!!!  When I added up all the sub-folders totals via Get Info, they matched exactly what Get Info proclaimed for folder #2!
Again, don't know why this happened, but I was able to successfully back up everything, and have had no troubles in the week and a half since.
If any of this doesn't make sense, please feel free to ask questions.  But this is how I solved the problem.