I haven’t played around with SD adapters but there is a common problem with compact flash cards that gives the same symptom. They won’t be bootable until you fire up DOS 6.22 and run FDISK /MBR, which is an undocumented command which gives no output but fixes the boot record. I would focus on getting the floppy drive working or buy a gotek so you can boot floppy images from USB and get into DOS that way.