Future ASICs will support 66 MHz operation. Data is passed through the FX100 board without being stored. Write to the first location of each 128k block, then read it back. When the data you read back isn't what you wrote, you have reached the end of the cards memory. Yes, as long as it has a FC connector.
Some drives require an intermediate adapter.
Seagate drives require an adapter that converts their proprietary 80 pin connector to a 1x3 differential connector. No, the application informs the driver that it wants to transfer a certain number of MBs, then the driver partitions and DMAs the data to the drive through the FibreXpress card's buffer. When the transfer is complete, the driver informs the application. All intermediate transactions are transparent to the application. They are assigned when the driver is installed.