The limiting factor of transfer rates right now is basically the drives IMO. We need to get past 7,200 RPM, which thankfully WD did for us a couple of days ago (they are going to begin selling 10,000 RPM IDE drives, 10K was previously only reserved for SCSI).
The interface for drives is a limiting factor, but no harddrive company is "unlazy" enough to basically almost eliminate the interface overhead. This would require making a new interface, and one which motherboards would have to be equipped with (otherwise you would have to buy a controller card). MB manufacturers would be too lazy to do this as well unless this new upcoming interface was widely accepted as something that would sell.
Like you could make a new interface called "Super ATA 75" And if your harddrive's read speeds were almost constantly around 75MB per second, then you'd have an accurate statement.
Maxtor tried to do this a bit with the introduction of ATA 133 (on parallel), to try and get rid of the interface overhead as much as possible.
But the whole ATA 100 idea is just a big lie, the only way to get data transfer to nearly that level if through simultaneous transfer in RAID mode.
For a truthful and honest statement, they should call the current gerenation of IDE drives "ATA 50"
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Benchmarks don't lie