Within a year or two of the introduction of Ultra DMA modes in ATA/ATAPI-4, two new Ultra DMA modes were created. These new modes allowed data
transfer at 44 MB/s and 66 MB/s respectively (actually, the latter should really be 67
MB/s, since the number is really two-thirds of 100, but anyway...) The 44 MB/s speed never
caught on, but new drives appeared on the market implementing Ultra DMA mode 4 at 66 MB/s.
Just as the first 33 MB/s Ultra DMA mode drives were dubbed Ultra
ATA/33, the new drives of course were called Ultra ATA/66 or ATA/66.
As with the original ATA/33, there is no real standard called "Ultra ATA/66";
this is a slang term for ATA drives using 66 MB/s Ultra DMA (UDMA mode 4). Still, this is
the common term given to drives that use the new Ultra DMA modes defined in the true
standard, ATA/ATAPI-5. In most cases you can assume that drives
sold as Ultra ATA/66 are compatible with ATA/ATAPI-5, though early "Ultra
ATA/66" drives may not include all the features defined in the formal standard.
Note: At some point in
your travels, you might also stumble upon some documentation that refers to the following
mysterious specification: "Ultra ATA/66+". See the
discussion of Ultra ATA/100 for the amusing story behind this term. 
Next: Ultra ATA/100
