Smack:
I don't remember exactly where I bought the adapter, but they were cheap and I got a couple of them. Someone here on the forum offered them for sale a little while back (Stone may have some squirreled away). I drilled a small hole in the SX's chassis support cross-member and attached the thing there with a self tapping machine screw (my SCSI also hangs perpendicular off that support bar), then just connected it up with a standard IDE HD cable. My experience is to get the best quality SD card as they are plentiful and cheap these days. An "Ultra" is a good example. The best thing about this primitive little PC is that you can run MFM, IDE, SCSI, and SD - maybe not all at the same time but 3 out of 4 ain't bad.
P.S. A 3.5" floppy occupies the space where the ST-225 used be; i.e. the "B" 5.25 floppy.