Fortunately I've fixed the problem, it was a defective RAM stick - Corsair Vengeance 1600MHz CL9. Though it passed many ram tests (Windows, Prime95, Aida, MemTest).
The original configuration had 16 GB RAM (4 x 4GB). After many tests I identified the defective stick. Now with only 8GB it works very smooth - no errors for 200GB archives tested. With the defective stick in it fails for 10GB.
On short:
- the defective stick passes many memory tests with no problem.
- if I insert it in the computer, it completely messes the data that the Z77 and/or Z68 HDD controllers operates on, introducing ugly HDD errors.
I'm glad it's not a motherboard chipset error after all.
The Z77-D3H seems pretty good and stable at first glance.