I have a problem with "Missing Operating system". First the screen with Gigabyte Motherboard takes too long. About 1-2 minutes. After that it shows me the next screen who is black with the error "Missing operating system". I mention that sometimes, if the Gigabyte screen takes 5-10 seconds it founds the hdd and problem is solved and the computer enter Windows. I don't understand why sometimes my hdd is found and sometimes not. What is the problem?
Well it could be one of the following problems:
My advice is to find your Windows 7 install DVD and boot from the DVD. Don't choose Install Now but choose Repair your Computer.
Here's a good tutorial on how to check your disk for errors and how to fix your MBR:
Just follow the steps in his post and your Computer should be fine. If none of the above work it's probably a hardware defect. If it's a hardware defect I would strongly suggest to backup all your DATA the next time you are able to boot into Windows 7.
Also what you can do the next time you are able to boot into Windows 7 is to check if your boot-drive is marked as active:
check this tutorial: How to Mark a Partition as Active in Windows 7
Another thing you could try is to simply restore your BIOS settings to its defaults (factory settings).
If you have any removable drives of USB drives plugged in - some motherboards have a hard time determining if they are bootable or not, and try to boot off of them. (I've ran into this personally)
You can determine this by unplugging any and rebooting. If the problem goes away, there's your issue. You can see if your bios settings have an option to ignore removable media.
Have you been taking hardware in and out. If so, make sure to check that your SATA cables are in their connectors straight and fully connected. Also make sure there are no kinks in the cables and they are not pushing against a access panel. Even if all is well, you might try swapping cables from another SATA drive, or trying new ones if you have them lying around. It sounds crazy, but things could "look" 100% and still not be. HDD errors that are not explicitly called out can be very difficult to diagnose.
I personally know sometimes our computer won't start after working on it and just pushing SATA cables and making sure they are resting against the side of the case we put on sometimes fixes it. Also, our motherboard doesn't like recieving 5V from a powered USB and will not start correctly or recognize drives correctly ONLY SOMETIMES. Point is, things like what you are mentioning can sometimes be something very elusive. If it isn't something more obvious like MrSeeds link would find, then you have to start thinking "outside of the box".