When working with virtual disk images on macOS, you might encounter situations where you need to mount individual partitions exposed as block device files. While Linux provides the convenient mount -o loop option, macOS's hdiutil has limitations when dealing with raw block devices.

The error hdiutil: attach failed - not recognized occurs because hdiutil is designed primarily for disk image formats like DMG and ISO, not raw block devices. This becomes particularly apparent when using tools like vdfuse that expose virtual disk partitions as block devices.

Here are several methods to work with block devices on macOS:

1. Using vdfuse with FUSE for macOS

First, ensure you have FUSE for macOS installed:

brew install --cask macfuse
brew install osxfuse

Then use vdfuse to expose partitions:

vdfuse -f /path/to/your.vhd /mount/point

2. Mounting with hdiutil (for supported formats)

For formats that hdiutil does support, use:

hdiutil attach -imagekey diskimage-class=CRawDiskImage -nomount /path/to/image.vhd

3. Using Disk Utility

For VHD/VMDK files, you can sometimes use:

diskutil attachDisk /path/to/image.vhd

4. Creating a DMG wrapper

Create a DMG that references your block device:

hdiutil create -size 1g -fs HFS+ -volname "BlockDevice" -attach /dev/diskXsY

For NTFS partitions:

sudo mkdir /Volumes/NTFS
sudo mount_ntfs /dev/diskXsY /Volumes/NTFS

For FAT32 partitions:

sudo mkdir /Volumes/FAT32
sudo mount_msdos /dev/diskXsY /Volumes/FAT32

Always check the raw devices first:

diskutil list

For more detailed information:

diskutil info /dev/diskX

Here's a simple bash script to automate mounting block devices:

#!/bin/bash

VHD_PATH="$1"
MOUNT_POINT="$2"

# Create mount point if it doesn't exist
mkdir -p "$MOUNT_POINT"

# Use vdfuse to expose partitions
vdfuse -f "$VHD_PATH" "$MOUNT_POINT"

# Find the partition we want to mount
PARTITION=$(ls "$MOUNT_POINT" | grep Partition | head -n 1)

# Mount the partition (adjust filesystem type as needed)
sudo mount -t ntfs "$MOUNT_POINT/$PARTITION" /Volumes/vhd-mount

When working with large block devices:

• Use SSD storage for better performance
• Consider splitting large images into smaller chunks
• Monitor I/O performance with iostat 1

Remember that mounting block devices often requires root privileges. Always:

• Verify the source of your disk images
• Use read-only mounts when possible (-o ro)
• Unmount properly when done (diskutil unmountDisk)

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When working with virtual disk images (VHD/VMDK) on macOS, developers often need to access individual partitions contained within these files. While Linux's mount -o loop provides a straightforward solution, macOS requires different approaches due to its Darwin-based architecture.

The native hdiutil tool is designed for disk images (DMG/ISO) rather than raw block devices. When you encounter the "not recognized" error, it indicates the file doesn't conform to Apple's disk image format specifications.

# This won't work for raw block devices:
hdiutil attach /my/mountpoint/Partition1

Method 1: Using vdfuse with FUSE for macOS

First, install the required components:

brew install --cask macfuse
brew install gromgit/fuse/vd2fuse

Then mount your virtual disk:

vd2fuse /path/to/your.vhd /mount/point

Method 2: Creating a Disk Image from Block Device

Convert the block device to a format macOS understands:

dd if=/my/mountpoint/Partition1 of=partition.img bs=1m
hdiutil attach -imagekey diskimage-class=CRawDiskImage partition.img

Method 3: Using qemu-nbd (Advanced)

For developers working with QEMU images:

brew install qemu
qemu-nbd --connect=/dev/nbd0 image.qcow2
diskutil list  # Identify the partition
mount -t [fstype] /dev/nbd0s1 /mnt

When working with large virtual disks:

• Method 1 (vdfuse) offers best performance for frequent access
• Method 2 creates temporary files consuming disk space
• Method 3 provides direct block access but requires QEMU

If you encounter permission problems:

sudo chmod 755 /my/mountpoint/Partition1

For filesystem errors:

fsck_[fstype] /dev/diskXsY