diff --git a/Source/CBIOS/cbios.asm b/Source/CBIOS/cbios.asm
index 805ef457..e6690f72 100644
--- a/Source/CBIOS/cbios.asm
+++ b/Source/CBIOS/cbios.asm
@@ -1511,8 +1511,8 @@ DSK_MBR3:
 ;
 DSK_MBR4:
 	; IF BOOT FROM PARTITION, USE NEW SECTORS PER SLICE VALUE
-	LD	HL,16384		; NEW SECTORS PER SLICE
-	LD	(SPS),HL		; SAVE IT
+	LD	HL,16384	; NEW SECTORS PER SLICE
+	LD	(SPS),HL	; SAVE IT
 
 	; UPDATE MEDIA ID
 	LD	A,MID_HDNEW	; NEW MEDIA ID
@@ -1520,20 +1520,79 @@ DSK_MBR4:
 ;
 DSK_MBR5:
 	; ADJUST LBA OFFSET BASED ON TARGET SLICE
-	LD	A,(SLICE)		; GET SLICE, A IS LOOP CNT
-	LD	HL,(SEKLBA)		; SET DE:HL
-	LD	DE,(SEKLBA+2)		; ... TO STARTING LBA
-	LD	BC,(SPS)		; SECTORS PER SLICE
-DSK_MBR6:
-	OR	A			; SET FLAGS TO CHECK LOOP CNTR
-	JR	Z,DSK_MBR8		; DONE IF COUNTER EXHAUSTED
-	ADD	HL,BC			; ADD ONE SLICE TO LOW WORD
-	JR	NC,DSK_MBR7		; CHECK FOR CARRY
-	INC	DE			; IF SO, BUMP HIGH WORD
-DSK_MBR7:
-	DEC	A			; DEC LOOP DOWNCOUNTER
-	JR	DSK_MBR6		; AND LOOP
+	LD	A,(SLICE)	; GET SLICE, A IS LOOP CNT
+	LD	HL,(SEKLBA)	; SET DE:HL
+	LD	DE,(SEKLBA+2)	; ... TO STARTING LBA
+	LD	BC,(SPS)	; SECTORS PER SLICE
+DSK_MBR6:                       
+	OR	A		; SET FLAGS TO CHECK LOOP CNTR
+	JR	Z,DSK_MBR8	; DONE IF COUNTER EXHAUSTED
+	ADD	HL,BC		; ADD ONE SLICE TO LOW WORD
+	JR	NC,DSK_MBR7	; CHECK FOR CARRY
+	INC	DE		; IF SO, BUMP HIGH WORD
+DSK_MBR7:                       
+	DEC	A		; DEC LOOP DOWNCOUNTER
+	JR	DSK_MBR6	; AND LOOP
 DSK_MBR8:
+	; LBA OFFSET OF DESIRED SLICE IS NOW IN DE:HL
+	; NEED TO CHECK IF THE SLICE IS BEYOND CAPACITY OF MEDIA
+	; IF LBA_OFF + SPS >= DSK_CAP, ERROR!
+
+	; SAVE LBA_OFF
+	PUSH	DE		; MSW
+	PUSH	HL		; LSW
+	
+	; ADD SPS TO COMPUTE LBA_REQ
+	LD	BC,(SPS)	; SECTORS PER SLICE
+	ADD	HL,BC		; ADD ONE SLICE TO LOW WORD
+	JR	NC,DSK_MBR9	; CHECK FOR CARRY
+	INC	DE		; IF SO, BUMP HIGH WORD
+DSK_MBR9:
+	; SAVE CAP_REQ
+	LD	(CAP_REQ),HL	; LSW
+	LD	(CAP_REQ+2),DE	; MSW
+;
+#IFDEF PLTWBW
+	; GET DSK_CAP (DE:HL)
+	LD	B,BF_DIOCAP	; HBIOS DISK CAPACITY FUNC
+	LD	A,(SEKUNIT)	; DISK UNIT NUMBER
+	LD	C,A		; ... INTO C
+	RST	08		; HBIOS CALL (DE:HL = CAPACITY)
+#ENDIF
+;
+#IFDEF PLTUNA
+	; GET DSK_CAP (DE:HL)
+	LD	C,$45		; UBIOS DISK INFO FUNC
+	LD	A,(SEKUNIT)	; DISK UNIT NUMBER
+	LD	B,A		; ... INTO B
+	RST	08		; CALL UNA (DE:HL = CAPACITY)
+#ENDIF
+;
+	; SAVE DSK_CAP (DE:HL)
+	PUSH	DE		; SAVE DSK_CAP (MSW)
+	PUSH	HL		; SAVE DSK_CAP (LSW)
+;
+	; CHECK DSK_CAP >= CAP_REQ, CF SET ON OVERFLOW
+	; NO NEED SAVE ACTUAL RESULT
+	OR	A		; CLEAR CARRY FOR SBC
+	POP	HL		; DSK_CAP LSW
+	LD	DE,(CAP_REQ)	; CAP_REQ LSW
+	SBC	HL,DE		; DSK_CAP - LBA_REQ (LSW)
+	POP	HL		; DSK_CAP MSW
+	LD	DE,(CAP_REQ+2)	; CAP_REQ MSW
+	SBC	HL,DE		; DSK_CAP - LBA_REQ (MSW)
+;
+	; RESTORE LBA_OFF
+	POP	HL		; LSW
+	POP	DE		; MSW
+;
+	; ABORT ON OVERFLOW WITH ERROR!
+	JR	NC,DSK_MBR10	; IF NO OVERFLOW, CONTINUE
+	OR	$FF		; SIGNAL ERROR
+	RET			; DONE
+;
+DSK_MBR10:
+	; FINALIZE SLICE LBA
 	SET	7,D		; SET LBA ACCESS FLAG
 	; RESAVE IT
 	LD	(SEKLBA),HL	; LOWORD
@@ -1758,6 +1817,7 @@ CCPBUF		.DW	0		; ADDRESS OF CCP BUF IN BIOS BANK
 MEDID		.DB	0		; TEMP STORAGE FOR MEDIA ID
 SLICE		.DB	0		; CURRENT SLICE
 SPS		.DW	0		; SECTORS PER SLICE
+CAP_REQ		.DW	0,0		; LBA CAP REQUIRED FOR SLICE
 STKSAV		.DW	0		; TEMP SAVED STACK POINTER
 ;
 #IFDEF PLTWBW
diff --git a/Source/CPM3/diskio.z80 b/Source/CPM3/diskio.z80
index 08ee1554..8808751b 100644
--- a/Source/CPM3/diskio.z80
+++ b/Source/CPM3/diskio.z80
@@ -38,7 +38,7 @@
 	
 	extrn	?bnkxlt
 	
-	extrn	phex8, cout
+	extrn	phex16, phex8, cout, crlf, crlf2
 
 
     ; CP/M 3 Disk definition macros
@@ -600,6 +600,53 @@ boot7:
 	dec	a			; dec loop downcounter
 	jr	boot6			; and loop
 boot8:
+
+	; LBA offset of desired slice is now in DE:HL.
+	; Need to check if the slice is beyond capacity of media.
+	; If lba_off + sps >= dsk_cap, error!
+
+	; Save lba_off
+	push	de		; msw
+	push	hl		; lsw
+	
+	; Add sps to compute lba_req
+	ld	bc,(sps)	; sectors per slice
+	add	hl,bc		; add one slice to low word
+	jr	nc,dsk_mbr9	; check for carry
+	inc	de		; if so, bump high word
+dsk_mbr9:
+	; Save cap_req
+	ld	(cap_req),hl	; lsw
+	ld	(cap_req+2),de	; msw
+
+	; Get dsk_cap (de:hl)
+	ld	b,1Ah		; hbios disk capacity func
+	ld	a,(unit)	; disk unit number
+	ld	c,a		; ... into c
+	rst	08		; hbios call (de:hl = capacity)
+
+	; Save dsk_cap (de:hl)
+	push	de		; save dsk_cap (msw)
+	push	hl		; save dsk_cap (lsw)
+
+	; Check dsk_cap >= cap_req, cf set on overflow
+	; No need save actual result
+	or	a		; clear carry for sbc
+	pop	hl		; dsk_cap lsw
+	ld	de,(cap_req)	; cap_req lsw
+	sbc	hl,de		; dsk_cap - lba_req (lsw)
+	pop	hl		; dsk_cap msw
+	ld	de,(cap_req+2)	; cap_req msw
+	sbc	hl,de		; dsk_cap - lba_req (msw)
+
+	; Restore lba_off
+	pop	hl		; lsw
+	pop	de		; msw
+
+	; Abort on overflow with error!
+	jp	c,err_noslice	; slice too high, error exit
+
+	; Finalize slice lba
 	set	7,d			; set LBA access flag
 	ld	(lba),hl		; save new lba, low word
 	ld	(lba+2),de		; save new lba, high word
@@ -870,6 +917,7 @@ unit		db	0		; working disk unit num
 slice		db	0		; working slice num
 lba		dw	0,0		; working  lba
 sps		dw	0		; sectors per slice
+cap_req		dw	0,0		; lba cap required for slice
 mbrsec		ds	512		; MBR sector buffer
 dma		dw	0		; current DMA address
 bank		db	0		; HBIOS DMA bank
diff --git a/Source/ver.inc b/Source/ver.inc
index 0962c624..a6b9cd84 100644
--- a/Source/ver.inc
+++ b/Source/ver.inc
@@ -2,7 +2,7 @@
 #DEFINE	RMN	4
 #DEFINE	RUP	0
 #DEFINE	RTP	0
-#DEFINE BIOSVER	"3.4.0-dev.1"
+#DEFINE BIOSVER	"3.4.0-dev.2"
 #define rmj	RMJ
 #define rmn	RMN
 #define rup	RUP
diff --git a/Source/ver.lib b/Source/ver.lib
index 24dd9054..37db20ef 100644
--- a/Source/ver.lib
+++ b/Source/ver.lib
@@ -3,5 +3,5 @@ rmn	equ	4
 rup	equ	0
 rtp	equ	0
 biosver	macro
-	db	"3.4.0-dev.1"
+	db	"3.4.0-dev.2"
 	endm