there are several options.
1. Implement a TCP/IP stack and use the FTP protocol to transfer files to the sdcard. This method may seem cumbersome but since you have a ethernet port on the system a TCP/IP stack may be the best solution.
2. Instead of implementing TCP protocol, which is quite complicated, you could use UDP instead and make your own file transfer procedure.
I have developed a system that uses a SDCard and ENC28J60 driver IC interfaced to a PIC 18F2620 mcu, this system uses UDP for data transfers. To implement UDP connectivity you need to add other basic protocols like ARP,ICMP etc.
The system I developed connects via ethernet to a wireless router, I use a laptop to access the device (uses a static IP address) and download data from the SDcard.
The above solutions would be very easy to develop if you have ready made stack for the mcu you are using.
thanks
a