diff --git a/Source/HBIOS/sd.asm b/Source/HBIOS/sd.asm
index 30851796..94724bf2 100644
--- a/Source/HBIOS/sd.asm
+++ b/Source/HBIOS/sd.asm
@@ -434,11 +434,23 @@ SD_INITUNIT2:
 SD_PROBE:
 ;
 #IF (SDMODE == SDMODE_DSD)
-	LD	A,$03			; SET BIT 0 & 1
-	OUT	(SD_SELREG),A		; WRITE TO SELECT REG
-	IN	A,(SD_SELREG)		; READ BACK, BIT 1 IS ALWAYS 0
-	CP	$01			; ... SO SHOULD READ BACK AS $01
-	RET
+	; ON DSD, SELREG, BIT 0 IS READ BACK AS WRITTEN, BIT 1
+	; IS ALWAYS READ AS ZERO.  TO TEST FOR EXISTENCE, WE
+	; WRITE 00 AND MAKE SURE IT READS BACK AS 00, THEN WE
+	; WRITE 11 AND MAKE SURE IT READS BACK AS 01.
+	LD	C,SD_SELREG		; USE C TO ADDRESS PORT
+	XOR	A			; A := 0
+	OUT	(C),A			; SELREG := 0
+	IN	A,(C)			; READ SELREG BACK
+	AND	$03			; ISOLATE 2 LOWEST BITS
+	CP	$00			; BOTH BITS SHOULD BE 0
+	RET	NZ			; FAIL IF NOT
+	LD	A,$03			; SET 2 LOWEST BITS
+	OUT	(C),A			; DO IT
+	IN	A,(C)			; READ SELREG BACK
+	AND	$03			; ISOLATE 2 LOWEST BITS
+	CP	$01			; SHOULD READ BACK AS $01
+	RET				; RETURN W/ ZF SET AS NEEDED
 #ENDIF
 ;
 	XOR	A			; SIGNAL SUCCESS
@@ -1728,7 +1740,7 @@ SD_DPTRIMP:
 ; MSB<-->LSB MIRROR BITS IN A, RESULT IN C
 ;
 MIRROR:
-#IF (SDCSIOFAST)			; SLOW SPEED, LEAST CODE SPACE
+#IF (((SDMODE == SDMODE_CSIO) | (SDMODE == SDMODE_MK4)) & SDCSIOFAST)
 	LD	BC,MIRTAB		; 256 BYTE MIRROR TABLE
 	ADD	A,C			; ADD OFFSET
 	LD	C,A