(i'm italian so be patient if my english isn't perfect, Thanks.)
When i try to put the pc in stand-by mode (suspend to RAM) it works but when it goes in standby mode i hear "stek" from the hard disk.
It's like if i press and hold the power button to remove the power supply.

How can i fix this problem?

P.S. When i boot appears a message: ATA1 SOFTRESET FAILED. It's related to this problem?

Hi. There are three possible modes. Here are the common names

1. Standby - screen off, but psu and cpu still use power. Used to fool pc owners into thinking they were saving power, when they were still using at least half power. You don't want to bother with this.

2. Suspend - suspend operations to ram. Everything goes quiet but you still use some battery. Consumes very little power, but will run the battery down in time.

3.Hibernate - writes everything to the swap partition, and kills the box. You use no power. If you boot on the same kernel, it can resume.

What type of pc have you? Post the output from
cat /sys/power/state

Thank you for your reply, i mean the second definition:

2. Suspend - suspend operations to ram. Everything goes quiet but you still use some battery. Consumes very little power, but will run the battery down in time.

the command you request returns: "standby mem disk"

Suspend works fine, i only have the problem about the Hard Disk

The above means you can go to standby, suspend, or hibernate

Just to make sure try these in turn

sync; echo -n mem > /sys/power/state After the restart, try
sync; echo -n disk > /sys/power/state

You can also look at the o/p of
grep PM /var/log/messages

sync; echo -n disk > /sys/power/state at the end of all the messages returns "sda: stopping device" or something like that. Then it shutdown. Works fine.

sync; echo -n mem > /sys/power/state seems it doesn't stop SDA before shutting down. And with this command i hear the bad noise from the hard disk

It's probably a disk park. We don't think of those these days, but back in the day when 10mb was a huge disk you could never fill, every box used park the disk before power off because the heads wopuld rattle and destroy it if you didn't. They now have autoparking, i.e. heads go safe as the disk stops.

Use hibernate, and forget suspend. Too many problems. Just remember to let the sata disks stop before moving the box. My laptop has a hibernate script linked to the lid button, and to battery critical. I don't use suspend because of different issues (video) and I don't miss it.

Sorry, I didn't understand perfectly what you mean. Are you saying that this problem can't be fixed and using suspend without fixing it could destroy my hard disk? Did I understand correctly?

I tried Freebsd (freesbie) and it seems to work fine. Maybe i'll change CentOS with Freebsd

I use suspend very often and if CentOS doesn't support it i rather prefer use Windows

Well I don't have Centos. Centos correctly configured does support it. Are you prepared to configure it?

Rather than go to that bother on my own laptop, I use hibernate. What OS you use is a personal choice, as is the software you pay for.