Using SD card on Dell R720

Hello, I'm needing to format an sd card and apply an image to it. I have centos installed on my R720 and I have iDrac enterprise. I removed the original vFlash card and installed a generic raspberry pi SD card with 16 GB in there. When I list the devices on the server it shows as 2GB just like the vFlash. How can I use the full SD and not wipe out my iDrac? Thank you for any help.