Most thankful to both of you and I'll see to use those programs in the future. Now, the problem I have just solved it, and it was a hardware one. I had, some time ago, unplugged the power cable from on of the hards but had left the signal ribbon cable plugged in. Then, in the BIOS, I simply left the unplugged hard (primary slave) 'Not installed'.
After writing, I remembered (gkrellm showed an alert in the disk) the problem I had that took me to disconnect power from the slave disk (a recurrent click in the unit) and did two things: I also unplugged the ribbon cable from the slave and made the BIOS to scan the disk units. This has proved to be a remedy. I'm sorry to have bothered you without having given the problem a little more thought. Regards.