Smart update hangs about at 50%

[flapane]

Hi,
500gb eide western digital.
MBP 10.5.6
Airport extreme n.

I first did the initial backup using an usb box connected to the laptop, using a sparseimage.
Then I connected the disk to Airport and starting doing a smart update of the lan sparseimage.

Now the hard part...
When I had a 160gb external hd, the initial backup and the next differential ones were smooth.
After I bought a 500gb it started having this kind of problem, no matter if the HDD is connected via airport or via usb.
It hangs something between 230000 examined files on 320000 (60GB in total).

I did a long bad sector test and I am sure that the hdd is perfect.
The hdd has 3 hfs+ journaled partitions.

any hints?
thanks in advance