When working with ZFS storage pools, many administrators encounter the same fundamental limitation when trying to expand existing RAIDZ vdevs. The error message you're seeing:

invalid vdev specification: raidz requires at least 2 devices

reveals a core architectural constraint in ZFS. Let's examine why this occurs and explore practical solutions.

The root issue stems from how ZFS handles vdev types and their minimum requirements:

• RAIDZ1/2/3: Requires minimum of 2+N disks (where N is parity level)
• Mirror: Requires minimum 2 disks
• Stripe: Can be created with single disk

Your attempt to add a single disk to RAIDZ violates these fundamental rules. Here are your actual options:

Option 1: Add as Separate VDEV

The most straightforward approach is adding the new disk as a separate vdev:

zpool add backupz c7t4d0

This creates a single-disk vdev in your pool. While functional, it lacks redundancy.

Option 2: Convert to Mirror (Recommended for Small Pools)

For better redundancy with minimal additional disks:

zpool attach backupz c7t0d0 c7t4d0

This converts your existing disk into a mirrored pair.

Option 3: Full Restructure (Best for Large Pools)

For optimal performance and redundancy:

1. Create new pool with desired structure
2. Use zfs send/receive to migrate data
3. Destroy old pool and rename new one

zpool create newbackup raidz1 c7t0d0 c7t1d0 c7t2d0 c7t3d0 c7t4d0
zfs snapshot -r backupz@migrate
zfs send -R backupz@migrate | zfs receive -F newbackup
zpool destroy backupz
zpool rename newbackup backupz

Each approach has distinct performance characteristics:

For repeated operations, consider this bash script template:

#!/bin/bash
# ZFS pool expansion helper
POOL="backupz"
NEWDISK="c7t4d0"

zpool status $POOL > /dev/null || exit 1

echo "Select expansion method:"
echo "1) Add as single disk"
echo "2) Convert to mirror (select existing disk)"
echo -n "Choice [1-2]: "
read choice

case $choice in
  1) zpool add $POOL $NEWDISK ;;
  2)
    echo "Available disks in pool:"
    zpool status $POOL | grep ONLINE | grep -v raidz
    echo -n "Select disk to mirror: "
    read existing
    zpool attach $POOL $existing $NEWDISK
    ;;
  *) echo "Invalid choice" ;;
esac

When working with ZFS storage pools, it's crucial to understand that RAIDZ vdevs have specific requirements for device counts. The error message you encountered - "raidz requires at least 2 devices" - reveals a fundamental constraint in ZFS architecture.

# This will fail:
zpool add backupz raidz c7t4d0
invalid vdev specification: raidz requires at least 2 devices

ZFS treats RAIDZ vdevs as atomic units. Unlike traditional RAID systems where you might expand arrays disk-by-disk, ZFS requires you to:

• Add complete vdevs to a pool
• Maintain RAIDZ parity requirements
• Keep vdev structure consistent

Here are your viable options with example commands:

# Option 1: Add the disk as a separate non-redundant vdev
zpool add backupz c7t4d0

# Option 2: Create a new mirrored pair (requires 2 disks)
zpool add backupz mirror c7t4d0 c7t5d0

# Option 3: Backup, recreate pool with larger RAIDZ
zfs send -R backupz@snapshot | zfs receive newbackupz
zpool destroy backupz
zpool create backupz raidz1 c7t0d0 c7t1d0 c7t2d0 c7t3d0 c7t4d0

Each approach has distinct performance characteristics:

For frequent pool expansions, consider this bash script:

#!/bin/bash
POOL="backupz"
NEWDISK="c7t4d0"

if [ ! -e "/dev/rdsk/${NEWDISK}" ]; then
    echo "Disk ${NEWDISK} not found"
    exit 1
fi

# Check if we can add as single disk
read -p "Add as non-redundant disk? (y/n) " choice
if [ "$choice" = "y" ]; then
    zpool add ${POOL} ${NEWDISK}
else
    echo "Please prepare a second disk for mirroring"
fi