this is directly copied from the boot9strap 1.4 release changelog on github, figured you might as well see it:
If the ntrboot combo (X+Start+Select) is held, or if either FIRM file is corrupt, the notification LED will display the following:
SD...
it is a batch apk file that contains extra apks for config of different screen sizes, one of them is going to be for your architechture as well, anyways, you can use this app to install the single file (the .apks file)...
don't take this as advice, seriously, don't, but could the ribbon cables be a problem? i know that it probably isn't the issue because the 2ds has no hinges, but even if it doesn't, they could still be coming loose, right? again, correct me if...
try replacing the boot.3dsx and boot.firm on the sd card with those that come with the latest version of luma
1711077264
i think you should also try checking both sd cards with h2testw, if the old one was failing than the files are likely...