I updated OS2LVM.DMD and LVM.DLL. Didn't help.
I agree, that it is possible, that it is something that changed between ACP1 and ECS, although it's hard to find what. I'm not sure what's important.
I didn't try another pendrive - I have no access to. But I guess that if it works perfectly on ECS on one machine, it should work on other... Or maybe USB of my old mainboard is not handled that well? Definetely it is in 1.0 speed on the ACP1 machine, and 2.0 speed on ECS... But that should not make a stability difference...
I'll keep looking and thanks you all for ideas.