diff --git a/README.md b/README.md
index 28467a6..5fa213c 100644
--- a/README.md
+++ b/README.md
@@ -9,7 +9,7 @@ Portable emulator of an Apple II+. Written in Go.
 - Apple II+ with 48Kb of base RAM
 - Sound
 - 16 Sector diskettes in DSK format
-- ProDos hard disk (read only, just to support [Total Replay](https://archive.org/details/TotalReplay))
+- ProDos hard disk
 - Emulated extension cards:
     - DiskII controller
     - 16Kb Language Card
diff --git a/apple2Setup.go b/apple2Setup.go
index 1a8dcb3..cd80757 100644
--- a/apple2Setup.go
+++ b/apple2Setup.go
@@ -83,7 +83,7 @@ func (a *Apple2) AddHardDisk(slot int, hdImage string) {
 	c.loadRom(buildHardDiskRom(slot))
 	a.insertCard(&c, slot)
 
-	hd := loadHardDisk2mg(hdImage)
+	hd := openHardDisk2mg(hdImage)
 	c.addDisk(hd)
 }
 
diff --git a/cardHardDisk.go b/cardHardDisk.go
index 870e59a..4ac02bc 100644
--- a/cardHardDisk.go
+++ b/cardHardDisk.go
@@ -1,5 +1,7 @@
 package apple2
 
+import "fmt"
+
 /*
 To implement a hard drive we just have to support boot from #PR7 and the PRODOS expextations.
 See Beneath Prodos, section 6-6, 7-13 and 5-8. (http://www.apple-iigs.info/doc/fichiers/beneathprodos.pdf)
@@ -7,7 +9,8 @@ See Beneath Prodos, section 6-6, 7-13 and 5-8. (http://www.apple-iigs.info/doc/f
 
 type cardHardDisk struct {
 	cardBase
-	disk *hardDisk
+	disk  *hardDisk
+	trace bool
 }
 
 func buildHardDiskRom(slot int) []uint8 {
@@ -41,9 +44,13 @@ func buildHardDiskRom(slot int) []uint8 {
 
 	copy(data[0x80:], []uint8{
 		0xad, ssBase + 0, 0xc0, // LDA $C0n0 ; Softswitch 0, execute command. Error code in reg A.
+		0x48,                   // PHA
 		0xae, ssBase + 1, 0xc0, // LDX $C0n1 ; Softswitch 2, LO(Blocks), STATUS needs that in reg X.
 		0xac, ssBase + 2, 0xc0, // LDY $C0n2 ; Softswitch 3, HI(Blocks). STATUS needs that in reg Y.
-		0x18, // CLC ; Clear carry for no errors.
+		0x18,       // CLC ; Clear carry for no errors.
+		0x68,       // PLA ; Sets Z if no error
+		0xF0, 0x01, // BEQ $01 ; Skips the SEC if reg A is zero
+		0x38, // SEC ; Set carry on errors
 		0x60, // RTS
 	})
 
@@ -63,27 +70,31 @@ const (
 )
 
 const (
-	proDosDeviceNoError             = 0
-	proDosDeviceErrorIO             = 0x27
-	proDosDeviceErrorNoDevice       = 0x28
-	proDosDeviceErrorWriteProtected = 0x2b
+	proDosDeviceNoError             = uint8(0)
+	proDosDeviceErrorIO             = uint8(0x27)
+	proDosDeviceErrorNoDevice       = uint8(0x28)
+	proDosDeviceErrorWriteProtected = uint8(0x2b)
 )
 
 func (c *cardHardDisk) assign(a *Apple2, slot int) {
 	c.ssr[0] = func(*ioC0Page) uint8 {
+
 		// Prodos entry point
 		command := a.mmu.Peek(0x42)
-		//unit := a.mmu.Peek(0x43)
-		dest := uint16(a.mmu.Peek(0x44)) + uint16(a.mmu.Peek(0x45))<<8
+		unit := a.mmu.Peek(0x43)
+		address := uint16(a.mmu.Peek(0x44)) + uint16(a.mmu.Peek(0x45))<<8
 		block := uint16(a.mmu.Peek(0x46)) + uint16(a.mmu.Peek(0x47))<<8
-		//fmt.Printf("[CardHardDisk] Command %v on unit $%x, block %v to $%x.\n", command, unit, block, dest)
+		if c.trace {
+			fmt.Printf("[CardHardDisk] Command %v on unit $%x, block %v to $%x.\n", command, unit, block, address)
+		}
 
 		switch command {
 		case proDosDeviceCommandStatus:
 			return proDosDeviceNoError
 		case proDosDeviceCommandRead:
-			c.readBlock(block, dest)
-			return proDosDeviceNoError
+			return c.readBlock(block, address)
+		case proDosDeviceCommandWrite:
+			return c.writeBlock(block, address)
 		default:
 			panic("Prodos device command not supported.")
 		}
@@ -100,15 +111,44 @@ func (c *cardHardDisk) assign(a *Apple2, slot int) {
 	c.cardBase.assign(a, slot)
 }
 
-func (c *cardHardDisk) readBlock(block uint16, dest uint16) {
-	//fmt.Printf("[CardHardDisk] Read block %v into $%x.\n", block, dest)
+func (c *cardHardDisk) readBlock(block uint16, dest uint16) uint8 {
+	if c.trace {
+		fmt.Printf("[CardHardDisk] Read block %v into $%x.\n", block, dest)
+	}
 
-	data := c.disk.read(uint32(block))
+	data, err := c.disk.read(uint32(block))
+	if err != nil {
+		return proDosDeviceErrorIO
+	}
 	// Byte by byte transfer to memory using the full Poke code path
 	for i := uint16(0); i < uint16(proDosBlockSize); i++ {
 		c.a.mmu.Poke(dest+i, data[i])
 	}
 
+	return proDosDeviceNoError
+}
+
+func (c *cardHardDisk) writeBlock(block uint16, source uint16) uint8 {
+	if c.trace {
+		fmt.Printf("[CardHardDisk] Write block %v from $%x.\n", block, source)
+	}
+
+	if c.disk.readOnly {
+		return proDosDeviceErrorWriteProtected
+	}
+
+	// Byte by byte transfer from memory using the full Peek code path
+	buf := make([]uint8, proDosBlockSize)
+	for i := uint16(0); i < uint16(proDosBlockSize); i++ {
+		buf[i] = c.a.mmu.Peek(source + i)
+	}
+
+	err := c.disk.write(uint32(block), buf)
+	if err != nil {
+		return proDosDeviceErrorIO
+	}
+
+	return proDosDeviceNoError
 }
 
 func (c *cardHardDisk) addDisk(disk *hardDisk) {
diff --git a/hardDisk.go b/hardDisk.go
index 8386e4b..3dbe111 100644
--- a/hardDisk.go
+++ b/hardDisk.go
@@ -1,12 +1,14 @@
 package apple2
 
 import (
-	"bytes"
 	"encoding/binary"
+	"errors"
+	"io"
+	"os"
 )
 
 /*
-Valid for ProDos hard disks in 2MG format. Read only.
+Valid for ProDos hard disks in 2MG format.
 
 See:
 	https://apple2.org.za/gswv/a2zine/Docs/DiskImage_2MG_Info.txt
@@ -20,8 +22,9 @@ const (
 )
 
 type hardDisk struct {
-	data   []uint8
-	header hardDisk2mgHeader
+	file     *os.File
+	readOnly bool
+	header   hardDisk2mgHeader
 }
 
 type hardDisk2mgHeader struct {
@@ -40,45 +43,88 @@ type hardDisk2mgHeader struct {
 	LengthCreator uint32
 }
 
-func (hd *hardDisk) read(block uint32) []uint8 {
+func (hd *hardDisk) read(block uint32) ([]uint8, error) {
 	if block >= hd.header.Blocks {
-		return nil
+		return nil, errors.New("disk block number is too big")
 	}
-	offset := hd.header.OffsetData + block*proDosBlockSize
-	return hd.data[offset : offset+proDosBlockSize]
+
+	buf := make([]uint8, proDosBlockSize)
+
+	offset := int64(hd.header.OffsetData + block*proDosBlockSize)
+	_, err := hd.file.ReadAt(buf, offset)
+	if err != nil {
+		return nil, err
+	}
+
+	return buf, nil
 }
 
-func loadHardDisk2mg(filename string) *hardDisk {
-	var hd hardDisk
-
-	hd.data = loadResource(filename)
-
-	size := len(hd.data)
-	if size < binary.Size(&hd.header) {
-		panic("2mg file is too short")
+func (hd *hardDisk) write(block uint32, data []uint8) error {
+	if hd.readOnly {
+		return errors.New("can't write in a readonly disk")
+	}
+	if block >= hd.header.Blocks {
+		return errors.New("disk block number is too big")
 	}
 
-	buf := bytes.NewReader(hd.data)
-	err := binary.Read(buf, binary.LittleEndian, &hd.header)
+	offset := int64(hd.header.OffsetData + block*proDosBlockSize)
+	_, err := hd.file.WriteAt(data, offset)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func openHardDisk2mg(filename string) *hardDisk {
+	var hd hardDisk
+
+	hd.readOnly = false
+	file, err := os.OpenFile(filename, os.O_RDWR, 0)
+	if os.IsPermission(err) {
+		// Retry in read-only mode
+		hd.readOnly = true
+		file, err = os.OpenFile(filename, os.O_RDONLY, 0)
+	}
+	if err != nil {
+		panic(err)
+	}
+	hd.file = file
+
+	fileInfo, err := hd.file.Stat()
 	if err != nil {
 		panic(err)
 	}
 
-	if hd.header.Preamble != hardDisk2mgPreamble {
-		panic("2mg file must start with '2IMG'")
+	minHeaderSize := binary.Size(&hd.header)
+	if fileInfo.Size() < int64(minHeaderSize) {
+		panic("Invalid 2MG file")
 	}
 
-	if hd.header.Format != hardDisk2mgFormatProdos {
-		panic("Only prodos hard disks are supported")
-	}
+	readHeader(hd.file, &hd.header)
 
-	if hd.header.Version != hardDisk2mgVersion {
-		panic("Version of 2MG image not supported")
-	}
-
-	if size < int(hd.header.OffsetData+hd.header.Blocks*proDosBlockSize) {
+	if fileInfo.Size() < int64(hd.header.OffsetData+hd.header.Blocks*proDosBlockSize) {
 		panic("Thr 2MG file is too small")
 	}
 
 	return &hd
 }
+
+func readHeader(buf io.Reader, header *hardDisk2mgHeader) {
+	err := binary.Read(buf, binary.LittleEndian, header)
+	if err != nil {
+		panic(err)
+	}
+
+	if header.Preamble != hardDisk2mgPreamble {
+		panic("2mg file must start with '2IMG'")
+	}
+
+	if header.Format != hardDisk2mgFormatProdos {
+		panic("Only prodos hard disks are supported")
+	}
+
+	if header.Version != hardDisk2mgVersion {
+		panic("Version of 2MG image not supported")
+	}
+}