Please refer below link for details on rebuilding IDSDM [URL] As you have working SD card on SD2 slot you need to go to BIOS and ensure SD2 is Primary SD card first. Then you can replace SD 1 with new SD card and on boot select yes when asked for rebuild.    
Hafa Adai, i have 8GB SD Cards set to Mirror. The server was purchased directly from Dell. Lucked out as the warranty ended June 2019. From IDRAC page i got this failure message. What is the procedure please to replace SD1.  I'll get a matching or larger SD Card for the mirror. SD1 Fail SD2 Good I'm reading conflicting information, after power down and removal of power cords. 1. Swap the Good SD2 Card- place it in SD1 then the replacement card in SD2 slot. or 2. Simply insert the replacement card in slot SD1 leaving SD2 untouched and the rebuild will take place after confirming Yes on the bios post.   Thank you and Stay Safe!    
R630 IDSDM SD1 Internal 8GB Fail redundancy lost