First check the connections to the HDD. Reconnect the drive.

Some older systems may see the drive and classify it as a SCSI device if you are using a Serial ATA host adapter. This is normal even though this is not a SCSI disc drive. Many systems’ BIOS will not identify a Serial ATA drive connected to a PCI SATA host adapter. This is because a PCI SATA Host Adapter has its own BIOS which is used to identify hard drives connected to it which is separate from the BIOS of the computer.

So look in the bios for a SCSI drive option and enable it, reboot and your drive should work.

 

The BIOS SATA options are:
SATA Controller mode: Enhanced / Compatible
if I choose Enhanced there is an option to choose:
AHCI Configuration: Enabled / Disable
I've tried all the combination with no success
the thing is if I connect the old HDD, and insert the WIN 7 CD, the installation starts.
what am I doing wrong here?

 

Reset your BIOS to boot to the drive which is holding your Windows install CD... usually setup in <F2>, <F8>, <F10>, or <F12>, depending on system board or computer brand.
Then attempt a cold boot to the Windows or Recovery CD

 

The CD is already the first to boot. I've tried a cold boot and it didn't work either.
The only way for the installation to start is if I change the Type from "Auto" to "IDE Removable" but then Windows can't be installed on the drive (installation message)