Hi Andy,
I needed a large data storage system to log data off a scientific instrument. I did not care if it was USB, SD, MMC, CF etc but I could not use old faithful RS-232 as those ports are disappearing fast off laptops, plus relying on a laptop or desktop to log the data tied up that computer all day.

I looked at the SD/MMC from CompSys and bought their eval kit for SD/MMC. I also looked at the ALFAT line then decided on the VDIP1 Vinculum from FTDI.

Their eval board is about $US40 and gives you a simple TTL interface to read and write to USB memory sticks. By plugging in a common USB to MMC/SD/Memory Stick/Compact Flash card reader/writer you immediately get access to most of the other card formats.

In its simplest form, the UART mode, the VDIP1 lets you read and write at 9600 bps 8N1 serial ascii over a 4 wire interface. Txd, Rxd, RTS & CTS. By going to a parallel interface the speed goes up to 300 kbps and there are other modes up to at least 4 MBps.

Have a look in Code Examples for the code I wrote to get started.

HTH
Brian