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Tell us the make and model of the used brand name system, or the make and model of the used system's mboard if it's a generic desktop system.
If the hard drives are IDE....
"Did you move the cables around?"
The correct end connector on an 80 wire IDE data cable must be connected to the IDE header on the mboard or to the IDE header on the drive controller card in a slot. Usually it's blue, but in any case, it's the one farther from the middle connector on a 3 connector data cable.
Hard drives usually use 80 wire data cables, but some optical drives can use 40 wire ones, if it's the only drive on a data cable, or if both drives on a data cable can use 40 wire cables.
Some 40 wire data cables can be plugged in either way on the IDE headers and on the drives. If one end of a connection is plugged in backwards, the other end is not, the drive will not be harmed, but it will not be recognized. The stripe on one side of the data cable connection should be next to the power connector on the drive, and the other end should be on the pin one end of the mboard or card IDE header, which is usually marled on the mboard or the card with a 1 or an arrowhead,
or - both ends can be plugged in the opposite that way.
"Did you change any jumpers on any of your drives?"
The jumpers on the back of the drive where the cables plug in must be set correctly.
The info about the hard drive jumper settings is on the label on the top of the drive, or you can look up the jumper info online by using the model number of the hard drive on the manufacturer's web site. Sometimes the label is upside down in relation to the back of the drive. If that label shows where the power connector is, set your jumper in relation to that.
Many IDE hard drive models, e.g. many Western Digital ones, have two ways they can be set to Master - e.g. Master, single, for when it is by itself on a data cable, and Master, with slave, for when another drive is on the data cable and is set to Slave. That must be correct for the situation.
On optical (CD or DVD) drives, usually there are markings near the jumpers, but they may merely be etched into the plastic and you may need good lighting to make them out.
Master - MA
Slave - SL
Cable Select - CS
If there is one drive on an IDE data cable, it should be set to Master, or be set to Cable Select and be on the end connector of a three connector data cable.
If there are two drives on the same IDE data cable, one must be set to Master, the other one to Slave,
or both drives must be set to Cable Select.
"...I get 'hard drive not found' "
If both drives are on the same data cable, and both jumpers on the drives are set to Master, or both jumpers are set to Slave, neither drive will be detected by the bios.
"....now has only the original drive in it, and I'm still getting the error of drive not found."
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.
"I tried to put the drive from my old computer into it."
If you install a drive that has had XP or 2000 installed on it when it was connected to one motherboard, then move that hard drive without doing anything to the data on the hard drive to another computer that has a different mboard, if you try to boot from that hard drive, if the hardware on the two mboards is more than a little different, XP and 2000 will often NOT load normally. The hard drive boots fine , but typically you see the first bit of Windows graphics, then a black screen with a blinking cursor top left, nothing further happens, or the operating system will reboot the system before it has fully loaded, etc.
That can be fixed without losing the data already on the partition Windows was installed on, which is usually C in Windows, by running a Repair installation procedure - that requires you have an operating system CD of the same version as on the hard drive, and a suitable Product Key, to fix the situation.