Does the macbook use the internal harddrive when booting from usb and when fully booted?

It even locks up when doing the extended hardware test. Usually between 7 and 11 minutes into it, when doing memory iterations. Doesn't mean it fails on the memory, it just freezes, just like when in the OS. I guess theres nothing left than bring it to the apple store. Although I do not wish to pay 400+ euro for a new logic board (it the problem is located there). Making them check for the problem costs about €79,- if I do not agree with the repair costs, so it's a tricky one. Anyway, thanks for the help guys.

Heres an update;

I went to an apple certified reseller, and the technician there did a check up on my macbook air. All the hardware seemed to be O.K. with the 'MRI' system check they did. After that he did an extended harddrive test with Disk Warrior and it froze up. So his conclusion was a broken SSD. He said replacing an SSD costed €700 ex. labor.

At home I cloned the SSD to an external harddrive using Carbon Copy Cloner, and removed the SSD from the macbook. Then I booted using my external harddisk via USB. Guess what? It still froze up. I also had a bootable USB and triple checked, again it froze up. So my conclusion is that the SSD must be fine. The apple technician drew his conclusions too quickly.

Since then I have put the SSD back in, and it seems to run more stable than before (weird). It doesn't freeze up every 2-5 minutes, rather it freezes up every 30min - 7 hours (top 7 hour heavy use without a crash). So I'm back to zero.

I am a little suspicious about the battery though. iStat Pro tells me the amperage, and connected to its charger it float around 1.65 Amps, running on its battery alone it goes way below that. Could the battery make the system unstable?

The ram is the problem. Goodbye Macbook Air. Great it is soldered onto the logic board, now the whole laptop is completely useless!