Re: Powerdege 2900 - hard disk blinking amber
1. They work like normal hard drives, but the difference is that fact that they are connected to a RAID/storage controller in the system and not directly to the motherboard. The controller manages the disks individually in an array that is presented to the OS and is all the OS can see. Your three drives MAY be in a RAID 5, meaning that you can lose one drive and the data will stay accessible; two drives failing means the data goes offline.
2. No. Unless someone just 'laid' a disk inside the box (there is no place to mount a drive internally), there is no way to put a drive in the system not attached through the backplane (drives accessible from outside). Those are your drives.
3. Not relevant. It is important to address a failed disk, regardless of its position in the system. How much time do you have? Maybe 5 minutes, maybe 2 years, it is completely dependent on when a second disk decides to go offline and can appear completely random. Replace the failed drive as soon as possible.
4. If ONE more disk fails, you lose your array and you may or may not be able to resurrect it, which would mean replacing all disks and restoring from backup or paying thousands to recover.
Replace it as soon as possible; replace it with a Dell-certified drive (same size or larger); replace it "hot" (do NOT power down to replace it).