Second HDD problem

Hi i think i have strange problem,

I have 2 x WD My Passport Essential HDDs 640 GB ubs 2.0

If I plug them to the player usb inputs and I select List All Video then only the content of one HDD gets listed the second HDD does not show the thumbnail jpegs only the default labels of videos. And I cant play the movies from this second hdd either. (it says error does not support the file format)

If I select the folder view I can choose one or the other HDD and the labels get listed and the movies work. But Id like to use All Video instead. Its faster.

If I use an other HDD, for example 1 x 640 GB and 1 x 320 GB WD External its ok. The content is merged and displayed together. All videos work.

I tried to delete .wd folder (from both) to reindex but did not help.

So what do you think? Is there a limitation of the 2 HDD capacity together?