SO here is the situation: First this was my old PC, it had a 2x 1TB RAID 0 and a Corsair Force 3 SSD in it. This were the old speeds, measured by HDTune Pro.

2x 1TB RAID 0:

Corsair Force 3 SSD

Then my dad got my PC and we had several issues, in the end turned out both RAID and SSD controller were malfunctioning causing BlueScreens on 100% load. Removed the RAID 0, but leaving the HDD's intact and bought an Samsung 840 EVO 120GB, though the Corsair SSD is still in the system, just not as sytem disk anymore.

1TB HDD (one of them):

Corsair SSD:

Samsung SSD:

We did not assemble the PC ourselves, so answering some technical questions might be more difficult, though we will do our best.

First thing we noticed is that the Samsung 840 EVO is no where reaching it's advertised speed, even an Samsung 840 250GB (non-EVO) is reaching 350 MB/s in my own PC.

Then we noticed that both SSD's are capped at 120 MB/s exactly, not sure if this is being caused by HDTune Pro, but very unlikely. And even worse, the Corsair Forza 3 was running faster before the system got reassembled.

Does anyone have any clue what is going on?