Windows 7: external HDD (USB 2) malfunction

My Spec:
Windows 7 Prof 64bit, SP1
Core Duo E7300
4 Gig RAM
4 onboard USB, 2 via PCI card
1,5 TB Samsung HD, extra power supply
.5 TB Samsung, extra power supply

At first only the 1.5 HD (and just right now) made some problem, slowing down extremley till its freezing all processes involving accessing the hd ( explorer.exe).
But shes running fine normally, no funny sounds, smart tells me all's a ok , on my laptop (Windows 7 prof ) or my brothers mac its running fine too.



Meanwhile while I tested the 1,5 at my brothers, the 500gig hd showed the same behavior !!, but not since im running both again.

I switched them both thru all 6 available usb ports, same behavior. (all other USB devices working fine! )
I reinstalled my Windows yesterday, ssdd