Audit Volume OK but Move Data FAILED (unreadable data) why ?

I don't understand this ....

08/05/2013 15:36:46 ANR4133I Audit volume process ended for volume Y15181;
1410 files inspected, 0 damaged files found and marked as
damaged, 0 files previously marked as damaged reset to
undamaged, 0 objects need updating. (SESSION: 452417,
PROCESS: 46
08/05/2013 15:36:46 ANR0515I Process 468 closed volume Y15181. (SESSION:
452417, PROCESS: 46
08/05/2013 15:36:46 ANR0987I Process 468 for AUDIT VOLUME (INSPECT ONLY)
running in the BACKGROUND processed 1,410 items with a
completion state of SUCCESS at 15:36:46. (SESSION:
452417, PROCESS: 46

470 Move Data Volume Y15181 (storage pool Y-CNPI-PROD_PP),
Target Pool Y-CNPI-PROD_PP, Moved Files: 0,
Moved Bytes: 0, Unreadable Files: 20, Unreadable
Bytes: 5,368,294,018. Current Physical File
(bytes): 1,858,369,847 Current input volume:
Y15181. Current output volume(s): Y15043.



If someone can explain me that, I will be grateful ....

Thanx.