Hello
First of: Thank you all for your help.
I finally decided to go radical, the motherboard/processor/memory where OK since it didn't failed at all at home after 2 days, so I buy a brand new power supply, case, and one 1TB hard drive, and it's working perfectly so far with SME 7.5
So, It has something to do with the power supply or the disks, I will dedicate this weekind to find out, since my client is now happy with his new working server (well... i didn't tell him that the motherboard/memory was the same one

, he just saw a new enclosure and figure it out the rest by himself hehe), so now, there is no rush to figure it out what is going on.
I did change the PSU a couple of days ago for a ANTEC BASQ 450w, that i think, is a very good PSU, remember the motherboard is an ATOM dual core based, and 2 WD Green drives, power consumption is FAR below 100watts, and it failed after 8 hours in the same way (with only one drive connected), that's why I rule it out. (it's a good info BTW, never knew that smaller-than-necessary PSU can cause hangups like this, thank you).
I keep the new motherboard (intel D525MW with 2GB DDR3) so i will use it to run tests, will post here results later

(since my client wants RAID 1, but I will not take any chances, i will test both drives very well with SME here before go into the production server again)
thank you all
light