|You have not provided enough information.|
Tell us the make and model of your brand name system, or if you have a generic desktop system, the make and model of the mboard.
See response # 2.
So far, we're assuming you didn't mess with bios settings, or with the wiring connections, or jumpering of the drive(s) is it's(their) IDE, just before you had this problem.
"I test the hard disk in another system and is working"
Then there's probably nothing wrong with the drive itself, but it's possible you have a problem with the data on it, depending on what you mean by "I tested it".
As well as you getting the "operating system not found" message when you have a floppy that's not bootable in a floppy drive while booting, depending on bios Boot Order or similar settings, the samething can happen with some bioses if you have a USB flash drive plugged in while booting - USB flash drives are never bootable unless you deliberately make them bootable yourself.
If you HAVE been messing with bios settings, or with the wiring connections, or jumpering of the drive(s) is it's(they're) IDE, just before you had this problem....
If you have more than one hard drive, the one the bios detects first, or the one that you have specified it should detect first, must be bootable (have an operating system on it) - the bios will NOT attempt to boot from other hard drives if the first one is not bootable.
When you have more than one internal hard drive connected, you may need to change which hard drive the bios detects first, if the default settings do not detect a bootable hard drive, and if at least one of the hard drives is not bootable.
There is a list of hard drives somewhere in the bios settings
- (usually) in a separate list, usually near the Boot Order or similar settings - the hard drive you want to boot from must be the first one in the list.
- or (not as common) the Boot Order or similar settings have more than one hard drive listed - the hard drive you want to boot from must be the first one in the list.
If you have changed the wiring connections, or jumpering of the drive(s) is it's(they're) IDE, just before you had this problem....
You may not have the jumpers on IDE drives set right, or you may have a problem with a data cable, or you may have forgotten to plug in the power connector to a hard drive, or, if the mboard is older and it's a SATA drive your bios is not finding bootable, some older mboards have some SATA headers you can't boot a hard drive connected to them from.
If you have changed the wiring connections before this problem happened, or since you've been messing with data cables after this problem occurred...
It is common to un-intentionally damage IDE data cables, especially while removing them - the 80 wire ones are more likely to be damaged. What usually happens is the cable is ripped at either edge and the wires there are either damaged or severed, often right at a connector or under it's cable clamp there, where it's hard to see - if a wire is severed but it's ends are touching, the connection is intermittent, rather than being reliable.
Another common thing is for the data cable to be separated from the connector contacts a bit after you have removed a cable - there should be no gap between the data cable and the connector - if there is press the cable against the connector to eliminate the gap.
80 wire data cables are also easily damaged at either edge if the cable is sharply creased at a fold in the cable.
Try another data cable if in doubt.
Check your SATA data cables. The connector on each end should "latch" into the socket on the drive and on the mboard, or on the drive controller card - it should not move when you merely brush your hand against it near the socket - if it does, mere vibration can cause a poor connection of it - use another SATA data cable that does "latch", or tape the connector in place.
(There is a slight projection or bump on one side of the outside of the connector that "latches" it into the socket - it's easily broken off or damaged)
The same thing applies for the SATA power connection.