fix: allow Open() on external files and add OpenDiskImage API

README.md

@@ -85,12 +85,30 @@ func main() {
 - `(*DirItem).IterDir() ([]*DirItem, error)`
 - `(*DirItem).Open() (*FibStream, error)`
 - `(*DirItem).Properties() (PropertiesDictionary, error)`
+- `(*VBK).OpenDiskImage(path string) (*DiskImage, error)`
 - `(*VBK).DiscoverGuest() (*Guest, error)`
 - `(*Guest).Volumes() []*GuestVolume`
 - `(*Guest).DefaultIndex() int`
 - `(*GuestVolume).ListDir(path string) ([]GuestEntry, error)`
 - `(*GuestVolume).ReadFile(path string, limit int64) ([]byte, error)`
 
+## Disk Image Example
+
+```go
+disk, err := backup.OpenDiskImage("/vm/disk0.vmdk")
+if err != nil {
+    log.Fatal(err)
+}
+defer disk.Close()
+
+fmt.Printf("disk size: %d bytes\n", disk.Size())
+
+buf := make([]byte, 512)
+if _, err := disk.Read(buf); err != nil {
+    log.Fatal(err)
+}
+```
+
 ## Guest Volume Example
 
 ```go

dir.go

@@ -219,7 +219,7 @@ func (d *DirItem) ListDir() (map[string]*DirItem, error) {
 }
 
 func (d *DirItem) Open() (*FibStream, error) {
-	if !d.IsInternalFile() {
+	if !d.IsFile() {
 		return nil, fmt.Errorf("%w: %s", ErrIsDirectory, d.Name)
 	}
 	size, _ := d.Size()

reader.go

@@ -151,6 +151,74 @@ func (v *VBK) Get(p string, base *DirItem) (*DirItem, error) {
 	return item, nil
 }
 
+// DiskImage is a readable, closeable view of a virtual disk stored in the VBK.
+type DiskImage struct {
+	r       io.ReaderAt
+	size    uint64
+	pos     int64
+	closers []io.Closer
+}
+
+func (d *DiskImage) Read(p []byte) (int, error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	if uint64(d.pos) >= d.size {
+		return 0, io.EOF
+	}
+	limit := int64(d.size) - d.pos
+	if int64(len(p)) > limit {
+		p = p[:limit]
+	}
+	n, err := d.r.ReadAt(p, d.pos)
+	d.pos += int64(n)
+	return n, err
+}
+
+func (d *DiskImage) Size() uint64 { return d.size }
+
+func (d *DiskImage) Close() error {
+	var first error
+	for _, c := range d.closers {
+		if err := c.Close(); err != nil && first == nil {
+			first = err
+		}
+	}
+	d.closers = nil
+	return first
+}
+
+// OpenDiskImage opens the virtual disk at diskPath (a .vmdk, .vhd, or .vhdx entry
+// inside the VBK) and returns a DiskImage that streams the full logical disk content.
+// Multi-extent VMDKs are reassembled transparently.
+func (v *VBK) OpenDiskImage(diskPath string) (*DiskImage, error) {
+	item, err := v.Get(diskPath, nil)
+	if err != nil {
+		return nil, err
+	}
+
+	stream, err := item.Open()
+	if err != nil {
+		return nil, err
+	}
+
+	disk := guestDiskFile{Path: diskPath, Item: item}
+	locked := &lockedReadSeekerAt{r: stream}
+	virtualReader, _, virtualDiskSize, closers, err := openVirtualDiskReader(v, disk, locked)
+	if err != nil {
+		// Fall back to the raw FibStream if the disk format is not recognised.
+		size, sizeErr := item.Size()
+		if sizeErr != nil {
+			stream.Close()
+			return nil, sizeErr
+		}
+		return &DiskImage{r: locked, size: size, closers: []io.Closer{stream}}, nil
+	}
+
+	allClosers := append([]io.Closer{stream}, closers...)
+	return &DiskImage{r: virtualReader, size: virtualDiskSize, closers: allClosers}, nil
+}
+
 type SnapshotSlot struct {
 	vbk           *VBK
 	offset        int64