What format is the drive you're copying to?
Bootup holding CMD+r, or the Option/alt key to boot from the Restore partition & use Disk Utility from there to Repair the Disk, then Repair Permissions.
Then Repair Disk for all external drives.
How much free space is on each drive?
I am just copying files within my HDD, so it's the default Mac format.
I did repair permissions from the Disk Utility, however without booting from the Restore partition.
I'll try that as soon as possible. Thank you for your advice.
There is 400+ GB of free space on my hard drive, my iPhoto library is around 180 GB.
I have to mention that right now it just worked to copy the same library from an external drive to my mac, which is weird, but it doesn't seem to work within the same hard drive. I was trying to copy the library from Volumes/MobileBackups (local backups time machine does) to my Pictures folders, but it doesn't seem to work.
Recently, I have encountered the same problem.
It turns out that many users have the "preparing backup" issue, but I gave it some thought and remembered that the problem started to occure after moving my iPhoto folder to an external drive.
I have formatted my external Time Machine drive etc. etc. Tonight, it worked for 3 backups. Now, it is locked "preparing backup" again.
radu92, could you please explained how it worked out for you.
I had the same problem when trying to 'batch copy' all 20,000+ photos from a USB 3.0 external flash drive (1.5 Tb Western Digital) to the Time Capsule so I can start to use the WD drive to hold the Time Machine stuff, and the Time Capsule (ie router + HDD) to hold the family photos so my wife can see them on her computer with only one hop in the network.
When I pulled the whole "Our Pictures" folder over, it started to copy, but before it got to where it could estimate the amount of time it would take, I thought it might have stalled and clicked the little 'x' cancelling the job.
I then did a test-drive / proof of concept move of one tiny folder that has one tiny photo in it, and it moved over in a split second.
Then I started moving the folders one or two at a time, and they went just fine.
It seems to be a mis-match between what the computer thinks is reasonable (I'll spool up for the next hour, then work like a dog for 8 or 10 hours, then let you know when I've finished!) and what I think is reasonable (Tell me how far along you are within a few seconds or I'm going to question your ability to even do this job).
I'll bet if I left it sitting there long enough it would have finished the 'batch.' I am going to pay extra in time, in that there will be 'downtime' this way - the minutes it is sitting there with nothing to do between me moving a smaller package into the queue.