diff --git a/xCAT-test/unit/getinstdisk_selection.t b/xCAT-test/unit/getinstdisk_selection.t
new file mode 100644
index 000000000..fe3ea3bd4
--- /dev/null
+++ b/xCAT-test/unit/getinstdisk_selection.t
@@ -0,0 +1,134 @@
+#!/usr/bin/env perl
+use strict;
+use warnings;
+
+use FindBin;
+use File::Spec;
+use File::Temp qw(tempdir);
+use Test::More;
+
+my $repo_root  = File::Spec->catdir( $FindBin::Bin, '..', '..' );
+my $script_dir = File::Spec->catdir( $repo_root, 'xCAT-server', 'share', 'xcat', 'install', 'scripts' );
+
+# The RHEL 10 installer includes its own copy of the script, so both carry the
+# same selection order and both are exercised here.
+my @scripts = map { File::Spec->catfile( $script_dir, $_ ) } qw(getinstdisk getinstdisk.rhels10);
+plan skip_all => 'getinstdisk not found' if grep { !-r $_ } @scripts;
+our $script;
+
+sub slurp {
+    open( my $fh, '<', $_[0] ) or die "Unable to read $_[0]: $!";
+    my $c = do { local $/; <$fh> };
+    close($fh);
+    return $c;
+}
+
+# The script reads /proc/partitions and writes under /tmp, so each scenario
+# runs a copy with those paths moved into its own sandbox, and a stub udevadm
+# serves the device properties from fixture files.
+sub run_scenario {
+    my (%disk) = @_;
+    my $sandbox = tempdir( CLEANUP => 1 );
+    my $fixdir  = "$sandbox/fix";
+    my $bindir  = "$sandbox/bin";
+    mkdir $fixdir;
+    mkdir $bindir;
+
+    my $body = slurp($script);
+    $body =~ s{/proc/partitions}{$sandbox/partitions}g;
+    $body =~ s{/tmp/xcat\.install_disk}{$sandbox/xcat.install_disk}g;
+    $body =~ s{/tmp/xcat\.getinstalldisk}{$sandbox/xcat.getinstalldisk}g;
+    $body =~ s{/dev/md/Volume0}{$sandbox/md/Volume0}g;
+    $body =~ s{"/dev/xvda"}{"$sandbox/xvda"}g;
+    open( my $sh, '>', "$sandbox/getinstdisk" ) or die $!;
+    print $sh $body;
+    close($sh);
+
+    open( my $parts, '>', "$sandbox/partitions" ) or die $!;
+    print $parts "major minor  #blocks  name\n\n";
+    my $minor = 0;
+    for my $name ( sort keys %disk ) {
+        printf $parts "   8 %5d  524288000 %s\n", $minor++, $name;
+    }
+    close($parts);
+
+    for my $name ( sort keys %disk ) {
+        my %attr = %{ $disk{$name} };
+        open( my $props, '>', "$fixdir/$name.props" ) or die $!;
+        print $props "ID_WWN=$attr{wwn}\n" if $attr{wwn};
+        print $props "DEVTYPE=disk\n";
+        close($props);
+        open( my $attrs, '>', "$fixdir/$name.attrs" ) or die $!;
+        print $attrs qq{    ATTRS{size}=="1024000000"\n};
+        print $attrs qq{    DRIVERS=="$attr{driver}"\n} if $attr{driver};
+        my @models = $attr{models} ? @{ $attr{models} } : ( $attr{model} ? $attr{model} : () );
+        print $attrs qq{    ATTRS{model}=="$_"\n} for @models;
+        close($attrs);
+    }
+
+    open( my $udev, '>', "$bindir/udevadm" ) or die $!;
+    print $udev <<'UDEV';
+#!/bin/sh
+for a in "$@"; do
+    case "$a" in
+    --name=*) name=${a#--name=} ;;
+    esac
+done
+name=${name#/dev/}
+case "$*" in
+*--query=property*) cat "$FIXDIR/$name.props" 2>/dev/null ;;
+*--attribute-walk*) cat "$FIXDIR/$name.attrs" 2>/dev/null ;;
+esac
+exit 0
+UDEV
+    close($udev);
+    chmod 0755, "$bindir/udevadm";
+
+    local $ENV{FIXDIR}     = $fixdir;
+    local $ENV{PATH}       = "$bindir:$ENV{PATH}";
+    local $ENV{MASTER_IP}  = '';
+    system("sh $sandbox/getinstdisk >$sandbox/log 2>&1");
+    my $chosen = -r "$sandbox/xcat.install_disk" ? slurp("$sandbox/xcat.install_disk") : '';
+    chomp $chosen;
+    return $chosen;
+}
+
+# Every scenario asserts against both copies of the script.
+sub selects {
+    my ( $expected, $name, %disk ) = @_;
+    for my $candidate (@scripts) {
+        local $script = $candidate;
+        my $variant = ( File::Spec->splitpath($candidate) )[2];
+        is( run_scenario(%disk), $expected, "$name ($variant)" );
+    }
+    return;
+}
+
+# A direct attached disk wins over a RAID volume when both are present. The
+# RAID volume sorts first by name, so the choice comes from the driver group.
+selects( '/dev/sdb', 'the direct attached disk wins over the RAID volume',
+    sda => { driver => 'megaraid_sas' },
+    sdb => { driver => 'ahci' } );
+
+# A server with only RAID volumes still selects one.
+selects( '/dev/sda', 'a RAID volume is selected when nothing better exists',
+    sda => { driver => 'megaraid_sas' } );
+
+# A SAS host adapter loses to a direct attached disk, and still wins over a
+# driver with no group of its own.
+selects( '/dev/sdb', 'the direct attached disk wins over the host adapter',
+    sda => { driver => 'mpt3sas' },
+    sdb => { driver => 'ahci' } );
+
+selects( '/dev/sda', 'the host adapter wins over an unknown driver',
+    sda => { driver => 'mpt3sas' },
+    sdb => { driver => 'virtio_blk' } );
+
+# A driverless NVMe device still gets selected from the last group.
+selects( '/dev/nvme0n1', 'an NVMe device is selected from the last group',
+    nvme0n1 => {} );
+
+# No usable disk falls back to the documented default.
+selects( '/dev/sda', 'no disks fall back to the default' );
+
+done_testing();