diff --git a/toolkit/tools/imagecustomizerapi/mountidentifiertype.go b/toolkit/tools/imagecustomizerapi/mountidentifiertype.go
index 3f9ae357c..c5e45ade6 100644
--- a/toolkit/tools/imagecustomizerapi/mountidentifiertype.go
+++ b/toolkit/tools/imagecustomizerapi/mountidentifiertype.go
@@ -22,6 +22,8 @@ const (
 
 	MountIdentifierTypeOverlay MountIdentifierType = "overlay"
 
+	MountIdentifierTypeCustom MountIdentifierType = "custom"
+
 	// MountIdentifierTypeDefault uses the default type, which is PARTUUID.
 	MountIdentifierTypeDefault MountIdentifierType = ""
 )
diff --git a/toolkit/tools/pkg/imagecustomizerlib/partitionutils.go b/toolkit/tools/pkg/imagecustomizerlib/partitionutils.go
index 05c1b02f4..717b4ad5e 100644
--- a/toolkit/tools/pkg/imagecustomizerlib/partitionutils.go
+++ b/toolkit/tools/pkg/imagecustomizerlib/partitionutils.go
@@ -313,7 +313,8 @@ func findSourcePartitionHelper(source string,
 	var partitionIndex int
 
 	if mountIdType != imagecustomizerapi.MountIdentifierTypeDeviceMapper &&
-		mountIdType != imagecustomizerapi.MountIdentifierTypeOverlay {
+		mountIdType != imagecustomizerapi.MountIdentifierTypeOverlay &&
+		mountIdType != imagecustomizerapi.MountIdentifierTypeCustom {
 		partition, partitionIndex, err = findPartition(mountIdType, mountId, partitions)
 		if err != nil {
 			return imagecustomizerapi.MountIdentifierTypeDefault, diskutils.PartitionInfo{}, 0, err
@@ -378,6 +379,11 @@ func parseSourcePartition(source string) (imagecustomizerapi.MountIdentifierType
 		return imagecustomizerapi.MountIdentifierTypeDeviceMapper, deviceMapperValue, nil
 	}
 
+	_, isDeviceCustom := strings.CutPrefix(source, "/dev/disk")
+	if isDeviceCustom {
+		return imagecustomizerapi.MountIdentifierTypeCustom, "", nil
+	}
+
 	if source == "overlay" {
 		return imagecustomizerapi.MountIdentifierTypeOverlay, "", nil
 	}