Validation fails if USB destination

The validation following the backup fails everytime if I use one of my 6 (USB box) HD:

- Samsung HD502JI, SATA, 500Gb
- Seagate ST350063 0AS, SATA, 500Gb
- Samsung HD250HJ, SATA, 250Gb
- WD Initio WD1600AAJB-00J3A, PATA, 160Gb
- Maxtor Initio 6Y120L0, EIDE, 120Gb
- WDC WD32 02ABYS-02B7A0, SATA, 320Gb

while it always succeeds if I use my NAS:

- LaCie Network Space 2

I'm 1''% sure it isn't caused by my computer (HD error, etc) so I deduce it's because of ATI2015 (in fact ATI2013 doesn't fail): have I to suspect that ATI2015 doesn't work good and go back to use ATI2013?

On the other hand.. how can I solve this problem?