What to do when time machine seems stuck on "Preparing Backup"

Question

This issue is happenning on my macbook pro, mid 2010. My drive setup is that I have a partition which I dedicated to time machine backups. The total size of my drive is 500GB, and I partitioned about 100GB for time machine. The reason for that is I only needed a few space since I setup time machine to include only the system and the apps, and NONE of the libraries such as itunes or photos. This means that the only backup will be for the system, profiles, and the app. I have been running this for a few months andhas been doing OK until up to now.
My problem is that my time machine seems to get stuck on "Preparing Backup". The latest backup was done last Nov 8, 2011, whcih is about 11 days from now. I am also aware that the time machine drive has only about 200 mb space left, but I am not worried at all because I've always TM to delete the oldest backup to free up space. My oldest backup was still Sept 8, and I'm ok if that will be deleted.
So I'd like to find out why it still stuck on "Preparing backup" ,and I am wondering if deleting a backup can solve it. Can it? what do I do to solve this?
Also, I use my computer alot , so it is basically running for about 18 hours a day, but still time machine didn't quite work well. I have tried reparing the disk using disk utility, but that didn't help. 

Answer 1

Comments inline below.
Pondini wrote:
joshua raphael wrote:
This issue is happenning on my macbook pro, mid 2010. My drive setup is that I have a partition which I dedicated to time machine backups. The total size of my drive is 500GB, and I partitioned about 100GB for time machine. The reason for that is I only needed a few space since I setup time machine to include only the system and the apps, and NONE of the libraries such as itunes or photos.
You may want to revisit that strategy:
A backup on the same drive isn't very secure -- when (not if) the HD fails, you risk losing everything.
In most cases, if there's a problem with OSX the best way to fix it is to just install a fresh version.  But you can't do that from that backup.
Indeed!
Backing-up apps that way will also be useless in some cases.  3rd-party apps that came with their own installers put other files in other places (and sometimes "helper" apps as well).  Unless you know what and where they all are, and restore them, too, the restored app won't work properly, if at all.
I think you're confusing each other with terminology.  iTunes music and iPhoto pictures are NOT "Libraries", which has a specific meaning when it comes to OS X.    "Libraries", in OS X normally refers to /Libraries and ~/Libraries folders.  This is not where iTunes music and iPhoto pictures are stored. iTunes music and iPhoto pictures are stored within ~/Music and ~/Pictures, respectively.
I have tried reparing the disk using disk utility, but that didn't help.
Disk Utility changed a bit in Lion.  When you select an entire disk, it only repairs the partition map, not all the volumes.  You must repair each one separately.
Wow, good to know.  (What a stupid change to make, for backwards compatibility, if for no other reason!)
See #D1 in Time Machine - Troubleshooting to figure out why the backups are hanging.
Whew.  This editor makes it so hard to make comments inline.  The key thing when time machine seems stuck is patience.  Sometimes it needs to be left to run overnight.  Unfortunately, it doesn't log or report it's progress very well.
Opening Time Machine settings in Preferences plus a command like this left running in Terminal.app is how I monitor progress.  And even so, there is very little logging indicating progress. 
sudo tail -f /Volumes/Time Machine/Backups.backupdb/My\ Computer/2012-07-11-133352.inProgress/.Backup.363731632.496216.log
The parts in italics will need to be adjusted.  Tab completion makes this easy, e.g type
sudo tail -f /Volumes/
and hit tab, type the first few letters of the relevant volume name, hit tab...
Also, I have a hypothesis which needs testing. 
I know that Time Machine gets confused if one deletes large files from a backup - specifically, Time Machine doesn't recognize that the size of the backup has changed.
I know because this made restoring a thinned out time machine backup ("thinned out" by deleting files from within the GUI while inside the Time Machine) to a small SSD harder than it should have been.  I had to restore to a big disk and then clone that volume to the SSD - even though the SSD had enough space, Time Machine didn't think so; it didn't "understand" that it had been thinned.
My hypothesis is that Time Machine also gets confused if the Time Machine partition is resized. I shrunk a Time Machine partition (no it wasn't on my main disk!) and about that time, Time Machine stopped working.  After thinning the volume, it started working again.