diff --git a/weed/shell/command_volume_check_disk.go b/weed/shell/command_volume_check_disk.go
index ca7efa5d4..3f9859045 100644
--- a/weed/shell/command_volume_check_disk.go
+++ b/weed/shell/command_volume_check_disk.go
@@ -3,11 +3,13 @@ package shell
 import (
 	"bytes"
 	"context"
+	"errors"
 	"flag"
 	"fmt"
 	"io"
 	"math"
 	"net/http"
+	"strings"
 	"time"
 
 	"slices"
@@ -158,7 +160,7 @@ func (vcd *volumeCheckDisk) checkWriteableVolumes(volumeReplicas map[uint32][]*V
 					continue
 				}
 			}
-			if err := vcd.syncTwoReplicas(a, b); err != nil {
+			if err := vcd.syncTwoReplicas(a, b, true); err != nil {
 				vcd.write("sync volume %d on %s and %s: %v\n", a.info.Id, a.location.dataNode.Id, b.location.dataNode.Id, err)
 			}
 			// always choose the larger volume to be the source
@@ -282,62 +284,81 @@ func (vcd *volumeCheckDisk) shouldSkipVolume(a, b *VolumeReplica) (bool, error)
 	return true, nil
 }
 
-func (vcd *volumeCheckDisk) syncTwoReplicas(a *VolumeReplica, b *VolumeReplica) (err error) {
-	aHasChanges, bHasChanges := true, true
+// syncTwoReplicas attempts to sync all entries from a source volume replica into a target. If bi-directional mode
+// is enabled, changes from target are also synced back into the source.
+func (vcd *volumeCheckDisk) syncTwoReplicas(source, target *VolumeReplica, bidi bool) (err error) {
+	sourceHasChanges, targetHasChanges := true, true
 	const maxIterations = 5
 	iteration := 0
 
-	for (aHasChanges || bHasChanges) && iteration < maxIterations {
+	for (sourceHasChanges || targetHasChanges) && iteration < maxIterations {
 		iteration++
-		vcd.writeVerbose("sync iteration %d for volume %d\n", iteration, a.info.Id)
+		vcd.writeVerbose("sync iteration %d/%d for volume %d\n", iteration, maxIterations, source.info.Id)
 
-		prevAHasChanges, prevBHasChanges := aHasChanges, bHasChanges
-		if aHasChanges, bHasChanges, err = vcd.checkBoth(a, b); err != nil {
+		prevSourceHasChanges, prevDestHasChanges := sourceHasChanges, targetHasChanges
+		if sourceHasChanges, targetHasChanges, err = vcd.checkBoth(source, target, bidi); err != nil {
 			return err
 		}
 
 		// Detect if we're stuck in a loop with no progress
-		if iteration > 1 && prevAHasChanges == aHasChanges && prevBHasChanges == bHasChanges && (aHasChanges || bHasChanges) {
+		if iteration > 1 && prevSourceHasChanges == sourceHasChanges && prevDestHasChanges == targetHasChanges && (sourceHasChanges || targetHasChanges) {
 			vcd.write("volume %d sync is not making progress between %s and %s after iteration %d, stopping to prevent infinite loop\n",
-				a.info.Id, a.location.dataNode.Id, b.location.dataNode.Id, iteration)
+				source.info.Id, source.location.dataNode.Id, target.location.dataNode.Id, iteration)
 			return fmt.Errorf("sync not making progress after %d iterations", iteration)
 		}
 	}
 
-	if iteration >= maxIterations && (aHasChanges || bHasChanges) {
+	if iteration >= maxIterations && (sourceHasChanges || targetHasChanges) {
 		vcd.write("volume %d sync reached maximum iterations (%d) between %s and %s, may need manual intervention\n",
-			a.info.Id, maxIterations, a.location.dataNode.Id, b.location.dataNode.Id)
+			source.info.Id, maxIterations, source.location.dataNode.Id, target.location.dataNode.Id)
 		return fmt.Errorf("reached maximum sync iterations (%d)", maxIterations)
 	}
 
 	return nil
 }
 
-func (vcd *volumeCheckDisk) checkBoth(a *VolumeReplica, b *VolumeReplica) (aHasChanges bool, bHasChanges bool, err error) {
-	aDB, bDB := needle_map.NewMemDb(), needle_map.NewMemDb()
+// checkBoth performs a sync between source and target volume replicas. If bi-directional mode is enabled, changes from target are also synced back into the source.
+// Returns whether the source and/or target were modified.
+func (vcd *volumeCheckDisk) checkBoth(source, target *VolumeReplica, bidi bool) (sourceHasChanges bool, targetHasChanges bool, err error) {
+	sourceDB, targetDB := needle_map.NewMemDb(), needle_map.NewMemDb()
+	if sourceDB == nil || targetDB == nil {
+		return false, false, fmt.Errorf("failed to allocate in-memory needle DBs")
+	}
 	defer func() {
-		aDB.Close()
-		bDB.Close()
+		sourceDB.Close()
+		targetDB.Close()
 	}()
 
 	// read index db
-	if err = vcd.readIndexDatabase(aDB, a.info.Collection, a.info.Id, pb.NewServerAddressFromDataNode(a.location.dataNode)); err != nil {
-		return true, true, fmt.Errorf("readIndexDatabase %s volume %d: %v", a.location.dataNode, a.info.Id, err)
+	if err = vcd.readIndexDatabase(sourceDB, source.info.Collection, source.info.Id, pb.NewServerAddressFromDataNode(source.location.dataNode)); err != nil {
+		return true, true, fmt.Errorf("readIndexDatabase %s volume %d: %v", source.location.dataNode.Id, source.info.Id, err)
 	}
-	if err := vcd.readIndexDatabase(bDB, b.info.Collection, b.info.Id, pb.NewServerAddressFromDataNode(b.location.dataNode)); err != nil {
-		return true, true, fmt.Errorf("readIndexDatabase %s volume %d: %v", b.location.dataNode, b.info.Id, err)
+	if err := vcd.readIndexDatabase(targetDB, target.info.Collection, target.info.Id, pb.NewServerAddressFromDataNode(target.location.dataNode)); err != nil {
+		return true, true, fmt.Errorf("readIndexDatabase %s volume %d: %v", target.location.dataNode.Id, target.info.Id, err)
 	}
 
 	// find and make up the differences
-	aHasChanges, err1 := vcd.doVolumeCheckDisk(bDB, aDB, b, a)
-	bHasChanges, err2 := vcd.doVolumeCheckDisk(aDB, bDB, a, b)
-	if err1 != nil {
-		return aHasChanges, bHasChanges, fmt.Errorf("doVolumeCheckDisk source:%s target:%s volume %d: %v", b.location.dataNode.Id, a.location.dataNode.Id, b.info.Id, err1)
+	errStrs := []string{}
+	targetHasChanges, err = vcd.doVolumeCheckDisk(sourceDB, targetDB, source, target)
+	if err != nil {
+		errStrs = append(
+			errStrs,
+			fmt.Sprintf("doVolumeCheckDisk source:%s target:%s volume %d: %v", target.location.dataNode.Id, source.location.dataNode.Id, target.info.Id, err))
+	}
+	sourceHasChanges = false
+	if bidi {
+		sourceHasChanges, err = vcd.doVolumeCheckDisk(targetDB, sourceDB, target, source)
+		if err != nil {
+			errStrs = append(
+				errStrs,
+				fmt.Sprintf("doVolumeCheckDisk source:%s target:%s volume %d: %v", source.location.dataNode.Id, target.location.dataNode.Id, source.info.Id, err))
+		}
 	}
-	if err2 != nil {
-		return aHasChanges, bHasChanges, fmt.Errorf("doVolumeCheckDisk source:%s target:%s volume %d: %v", a.location.dataNode.Id, b.location.dataNode.Id, a.info.Id, err2)
+	if len(errStrs) != 0 {
+		return sourceHasChanges, targetHasChanges, errors.New(strings.Join(errStrs, ", "))
 	}
-	return aHasChanges, bHasChanges, nil
+
+	return sourceHasChanges, targetHasChanges, nil
 }
 
 func (vcd *volumeCheckDisk) doVolumeCheckDisk(minuend, subtrahend *needle_map.MemDb, source, target *VolumeReplica) (hasChanges bool, err error) {