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When I try to copy a CD, Why do I get an error that says my source and destination drives are on the same IDE bus?

What can I do about it?

by Tom Cumming


The IDE (Integrated Drive Electronics) bus is the interface most commmonly used for connecting storage devices to computers.

They consist normally of two connectors on the computer's motherboard, each of which can have a maximum of two storage devices connected to each. This does not include floppy disk drives: they have a seperate, narrower connector, that can also have a maximum of two drives of any mixture of sizes.

In the following diagrams,
===== represents an IDE connection;
----- represents a floppy disk connection.

When CD-ROMs first came in, in about 1994, a typical PC would be set up something like this:

IDE port 1 ====== Hard drive ====== CD-ROM drive
IDE port 2 == (not used)
Floppy port ------ Floppy drive

However, as the years went on, and hard drives got faster and faster, it was found that having the CD-ROM on the same cable as the hard drive, caused the hard drive to run more slowly than it could do. So to make their PCs faster, manufacturers started wiring things up like this:

IDE port 1 ====== Hard drive
IDE port 2 ====== CD-ROM drive
Floppy port ------ Floppy drive

This was fine, until CD-writers came in. Now, the manufacturers had two options. They could either do this:


IDE port 1 ====== Hard drive ====== CD writer
IDE port 2 ====== CD-ROM drive
Floppy port ------ Floppy drive

or (2)

IDE port 1 ====== Hard drive
IDE port 2 ====== CD-ROM drive ====== CD writer
Floppy port ------ Floppy drive

If you do (1), you have got the same problem you had before: your hard drive slows down. The amount that it slows down depends on the drive's data transfer rate: if the CD-ROM drive is UDMA 33 and the hard drive is UDMA 66, for example, then they will both be reduced to UDMA 33. However if they are both UDMA 66, then there is no problems.

Consider it like two trains, running in the same direction along a single stretch of track. The one at the front has a top speed of 70Mph, whereas the one at the back can do 100Mph. However the one at the back cannot actually go at 100Mph because it will crash into the one in front, doing 70Mph, so he also must do 70Mph. However, if the two trains are on seperate, parallel tracks, then the 100Mph train can overtake the 70Mph train.

However, if you have your PC wired up in arrangement (2), then you have eliminated the speed problem. However, you then introduce a new problem.
All data has to go via the motherboard, and the data can only travel down the wires in one direction at a time. Therefore, when you need to copy a CD, you get a situation a bit like when there are lanes closed on a road, and temporary lights are used. The traffic in one direction has to stop for a while to let the traffic come past in the other direction, then they
swap around.

Why does this matter ? Well, as the data is travelling from the CD-ROM to the CD-writer, if the data has to keep stopping and starting, it may mean that the data does not get to the CD-writer quick enough to be burned onto the CD, and so you get a "buffer-underrun" error and your CD is ruined.

So which is the best arrangement, (1) or (2) ? Well, it depends on your hardware. If you have a slow hard drive, that will not slow down your CD-ROM drive, then (1) is best. If however you have a fast hard drive, then (2) is best.

But, the problem here is that if you use (2) then you have still got the copying-CDs problem.

So, what should you do?

Firstly, try ignoring the error message and seeing if it will copy a CD anyway! If (like me) your CD-writer is fairly slow, then having the DVD and the CD-writer on the same cable, does not actually matter!

If you managed to produce a copy successfully, then you are OK.

If not, there are a few workarounds:

1) Slow down the burn speed. Then the data does not have to
travel as fast and there is more chance of it working.

2) Do not copy "on the fly". This means that the data on the original CD will be copied onto the hard drive first, and then onto the new CD afterwards. Again, slower but more reliable.

3) If you don't mind spending money, then you can buy an IDE controller card. This consists of a little PCI card that goes into one of the expansion slots inside your PC. It will give you some more IDE connectors. Then, you can arrange your PC like this:

IDE port 1 ====== Hard drive
IDE port 2 ====== CD-ROM drive
IDE port 3 ====== CD writer
Floppy port ------ Floppy drive

Then you have got rid of the disadvantages of both arrangements.


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