May I know is there any way can use the SD card slot on the DE1 SOC board as a external storage and allow the fpga to read the data from the sd card?
You can check this video on Design of Qsys system showcasing Nios II processor - SD card interface;
This video demonstrates 2 applications showcasing an Altera Nios II processor - SD card interface.
i) Shows creating, reading, and writing to a text file in a SD card
ii) Reads out a bitmap image from SD card and displays it on a VGA monitor
Let me know if you need extra information.