This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
There are several reasons why it is possible that an Ultra-ATA interface is no faster than an older one even if the drivers are installed and everything works properly. The first, and most important, is that the bandwidth of the fastest ATA-2 mode, 16.6MB/s, exceeds the real world transfer rate of most drives on the market today by a considerable margin. If this is the case for your drive, increasing the interface bandwidth beyond this will have no noticeable effect.
A further possibility is that your drive does not support the Ultra- ATA transfer modes at all. In that case, an Ultra-ATA interface may improve performance relative to an old-fashioned ISA bus or VL bus interface, but it will be no faster than a good PCI ATA-2 (EIDE) interface.