Not much. If you have only one drive on the cable, it is best to put it at the very end, especially when you're using any of the faster modes. For two devices, it doesn't matter where you put the master and the slave, or which end of the cable you plug into the controller. Just take care that you plug them in the right way: the red wire is supposed to correspond to pin 1.
When Plug'n'Play ATA arrives the Cable Select (CS) setting will be used, and all of this will change. Some name brand machines may already employ a similar setup.
This is not necessarily the case. Still, it is generally preferable to connect older drives and CD-ROMs to the secondary channel.
If this is not feasible, or if you're wondering if you should upgrade, a few points.
OS/2 and Unix users have another reason to put slow ATA devices such as tapes and CD-ROMs on a channel of their own. As long as one unit on a given channel is executing a command, the other is inaccessible. A CD-ROM can easily occupy the channel for 300ms that way.
Less than 18 inches. In some cases, the limit is no more than 7 inches.
The cable is a pretty weak link in the whole ATA-2 interface. For the fast transfer speeds used in 'EIDE' systems, there aren't enough ground signals; the cable is unterminated and unshielded. Noise is a real problem. For those reasons, you should take the 18" limit specified by the ATA(-2) standard pretty seriously if you want to avoid data corruption and system hangs.
Even worse, some dual-channel 'EIDE' interfaces such as CMD640x based ones have a peculiar design employing only a single buffer for both cables. Most signals on the two cables are directly connected with each other: this means that electrically, the cable lengths add up. Take into account that the copper traces on the circuit board are often a couple of inches long as well and you're facing a maximum cable length of roughly 7" per cable if you want to remain within spec.
If you have difficulties fitting everything in with a standard length cable, consider adding a new plug to it or rolling a complete cable yourself. It's not hard to do, or to track down a friend willing to do it, and you can buy the parts in all electronic parts stores. Do use quality parts, work carefully, and watch that length.
Disregard the above at your own peril. Noise induced problems usually pop up sporadically, can be very hard to detect and even harder to track down. Not least because they may appear completely unrelated and involve devices other than the harddisk.
Transferring a partitioned and formatted harddisk between computers is potentially dangerous if they use different translation schemes (see Q 6.3 ). This includes moving a disk from an old computer that doesn't support translation to a new one that does. This is really a little known issue. Usually everything goes smoothly, but you would not be the first to be caught by surprise.
WARNING. Do not assume it won't happen to you just because it works out fine most of the time---it has destroyed data, and will do so again.
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