diff --git a/run_helpers.go b/run_helpers.go
index fe8f331..0fc0eac 100644
--- a/run_helpers.go
+++ b/run_helpers.go
@@ -6,6 +6,9 @@ import (
 	"log"
 	"os"
 	"os/exec"
+	"path/filepath"
+	"strconv"
+	"strings"
 
 	"github.com/diskfs/go-diskfs/disk"
 	"github.com/diskfs/go-diskfs/partition/gpt"
@@ -66,7 +69,15 @@ func resizeFilesystem(
 	}
 	switch deviceType {
 	case disk.DeviceTypeBlockDevice:
-		return execResize2fs(device, newSizeMB, fixErrors)
+		// resize2fs takes the *partition* device, not the whole-disk
+		// device, so we resolve "/dev/sda" + partition number 9 to
+		// "/dev/sda9" (or whatever the kernel calls that slot —
+		// "/dev/nvme0n1p9", "/dev/mmcblk0p9", etc.) via sysfs.
+		partDevice, err := partitionDevicePath(device, filesystemData.number, "")
+		if err != nil {
+			return fmt.Errorf("cannot find partition device for %s partition %d: %w", device, filesystemData.number, err)
+		}
+		return execResize2fs(partDevice, newSizeMB, fixErrors)
 	case disk.DeviceTypeFile:
 		// copy the partition, then resize it, then copy it back into the original disk image
 		tmpFile, err2 := os.CreateTemp("", partTmpFilename)
@@ -158,3 +169,45 @@ func planResizes(
 	// recalculate resizes with shrinking
 	return calculateResizes(d.Size, table.Partitions, prTargetsWithShrink)
 }
+
+// partitionDevicePath maps a whole-disk path (e.g. "/dev/sda") and a
+// partition number to the partition's device path (e.g. "/dev/sda9",
+// "/dev/nvme0n1p9", "/dev/mmcblk0p9").
+//
+// Naming conventions for partition device nodes differ by disk type,
+// so we look up the kernel partition name via sysfs rather than
+// hardcoding the convention: each /sys/class/block/<disk>/<part>/
+// directory holds a "partition" file containing the partition number
+// and a directory named after the kernel partition name.
+//
+// If syspath is empty, /sys is used. Returns an error if no matching
+// partition is found under sysfs.
+func partitionDevicePath(diskPath string, partNumber int, syspath string) (string, error) {
+	if syspath == "" {
+		syspath = sysDefaultPath
+	}
+	diskBase := filepath.Base(diskPath)
+	diskSysDir := filepath.Join(syspath, "class", "block", diskBase)
+	entries, err := os.ReadDir(diskSysDir)
+	if err != nil {
+		return "", fmt.Errorf("read sysfs dir %s: %w", diskSysDir, err)
+	}
+	for _, e := range entries {
+		if !e.IsDir() {
+			continue
+		}
+		partFile := filepath.Join(diskSysDir, e.Name(), "partition")
+		raw, err := os.ReadFile(partFile)
+		if err != nil {
+			continue
+		}
+		n, err := strconv.Atoi(strings.TrimSpace(string(raw)))
+		if err != nil {
+			continue
+		}
+		if n == partNumber {
+			return filepath.Join("/dev", e.Name()), nil
+		}
+	}
+	return "", fmt.Errorf("partition %d not found under %s", partNumber, diskSysDir)
+}
diff --git a/run_helpers_test.go b/run_helpers_test.go
index eca3971..dcfe40f 100644
--- a/run_helpers_test.go
+++ b/run_helpers_test.go
@@ -4,6 +4,7 @@ import (
 	"errors"
 	"fmt"
 	"io/fs"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -194,3 +195,85 @@ func TestPlanResizes(t *testing.T) {
 		})
 	})
 }
+
+// TestPartitionDevicePath verifies that partitionDevicePath resolves
+// a whole-disk path + partition number to the kernel-named partition
+// device path via a sysfs lookup. Two fake-sysfs trees cover the
+// common naming conventions:
+//
+//   - sda → sda9 (the traditional convention used by most SATA/SCSI
+//     devices, where the partition number is appended directly).
+//   - nvme0n1 → nvme0n1p9 ("p" prefix before the number, used by
+//     NVMe, eMMC, and other devices whose name already ends in a
+//     digit so a bare "9" would be ambiguous).
+//
+// Hardcoding the convention based on the disk path is the wrong
+// approach (it gets mmcblk, nvme, virtblk, loop, and similar wrong),
+// which is why we use a sysfs lookup instead.
+func TestPartitionDevicePath(t *testing.T) {
+	tmp := t.TempDir()
+	sysClassBlock := filepath.Join(tmp, "class", "block")
+
+	// Set up fake sda with partitions sda1 and sda9.
+	for _, p := range []struct {
+		name string
+		num  string
+	}{
+		{"sda1", "1"},
+		{"sda9", "9"},
+	} {
+		if err := os.MkdirAll(filepath.Join(sysClassBlock, "sda", p.name), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(filepath.Join(sysClassBlock, "sda", p.name, "partition"), []byte(p.num+"\n"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	// Set up fake nvme0n1 with partitions nvme0n1p1 and nvme0n1p9.
+	for _, p := range []struct {
+		name string
+		num  string
+	}{
+		{"nvme0n1p1", "1"},
+		{"nvme0n1p9", "9"},
+	} {
+		if err := os.MkdirAll(filepath.Join(sysClassBlock, "nvme0n1", p.name), 0o755); err != nil {
+			t.Fatal(err)
+		}
+		if err := os.WriteFile(filepath.Join(sysClassBlock, "nvme0n1", p.name, "partition"), []byte(p.num+"\n"), 0o644); err != nil {
+			t.Fatal(err)
+		}
+	}
+
+	t.Run("sda partition 9", func(t *testing.T) {
+		got, err := partitionDevicePath("/dev/sda", 9, tmp)
+		if err != nil {
+			t.Fatalf("partitionDevicePath: %v", err)
+		}
+		if got != "/dev/sda9" {
+			t.Errorf("partitionDevicePath = %q, want /dev/sda9", got)
+		}
+	})
+	t.Run("nvme0n1 partition 9", func(t *testing.T) {
+		got, err := partitionDevicePath("/dev/nvme0n1", 9, tmp)
+		if err != nil {
+			t.Fatalf("partitionDevicePath: %v", err)
+		}
+		if got != "/dev/nvme0n1p9" {
+			t.Errorf("partitionDevicePath = %q, want /dev/nvme0n1p9", got)
+		}
+	})
+	t.Run("partition not found", func(t *testing.T) {
+		_, err := partitionDevicePath("/dev/sda", 42, tmp)
+		if err == nil {
+			t.Fatal("expected error for non-existent partition number")
+		}
+	})
+	t.Run("disk not found", func(t *testing.T) {
+		_, err := partitionDevicePath("/dev/sdz", 1, tmp)
+		if err == nil {
+			t.Fatal("expected error for non-existent disk")
+		}
+	})
+}