Why is my new Extreme IV Compact Flash card no faster than my Extreme III?
There are several factors that affect real world read and write speeds.
The most important is how the host device and the memory card communicate. There are two protocols that Compact Flash cards use to operate. The most common at this time is PIO (Programmed Input/Output), but more devices will begin to use the newer UDMA (Ultra Direct Memory Access). The Extreme IV card uses UDMA mode 4 to achieve the 45 MB/s rated speed. But most cameras these days can only communicate in PIO mode, so the Extreme IV must switch to PIO mode 6 (or lower), and that means that read and write speeds will be slower. The device manufacturer would be the best source to find out what protocol their device uses.
Some examples of the difference in speed between PIO and UDMA (these speeds are maximum theoretical speeds): PIO
Mode 6 - 25MB/s
Mode 4 - 16.7MB/s
Mode 3 - 11.1MB/s
Mode 2 - 8.3MB/s
Mode 1 - 5.2MB/s
Mode 5 - 100MB/s
Mode 4 - 66.7 MB/s
Mode 3 - 44.4MB/s
Mode 2 - 33.3MB/s
Mode 1 - 25MB/s
Again, if you are not seeing the speed that you are expecting, try to find out from the manufacturer what protocol your device uses. Remember, just because the card is capable of reading and writing data at a certain speed does not mean the camera or card reader is able to also.