Time Machine backing up slowly I hadn't backed up to Time Machine in 20 days, and now that I am trying to do so I have yet to succeed.

It got stuck a few times with a few KB; other times it took forever do everything, from preparing the backup to actually transferring files. It's supposedly trying to backup 40GB and after 8 hours not even 1 GB is done. I have to stop it because I have to take my 2009 Macbook running Lion out.

I ran Volume Repair on the external HD and everything came back fine. I also tried manually copy pasting a large video file to that volume and it copied quite fast. So must be something with Time Machine itself.

Can anyone help?

Speaking of 40GB I feel like that's way too much data that has changed over 20 days. I don't think that is correct. How can I check what is being backed up anyway?

Edit:

I ran tmutil. Final tally:

Added:         18.4G
Removed:       13.3G
Changed:       19.1G


Is this what TM means when it tells me it's backing up 40GB? Including stuff that will be removed?
I believe this is a staleness prevention.  That whenever there is too long between backups Time Machine uses a more thorough way to ensure that the backup is complete.

It is my understanding that the 40 GB is a "this is how much we consider for backing up" and that the actual data transferred may prove to be a lot less if the files turned out to still be identical.

Based on your description I believe that you have reached a point where you need to let your Time Machine backup catch up - work on its own until it is done, if at all possible.  If you cannot wait for that, perhaps now is a good time to buy a new drive and use that instead and have the old drive as a backup backup.