Create snapshot in virsh for domain with 2 or more disks

Question

I would like to backup the disks of a VM following this procedure https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit

1) I create an external snapshot with virsh snapshot-create-as --domain vm_name snap_name --diskspec vda,file=snapshot_folder/snap.qcow2 --disk-only --atomic --quiesce

2) Copy the base disk o the remote location

3) Commit the disk with virsh blockcommit vm_name vda --active --verbose --pivot

4) Delete the snapshot with virsh snapshot-delete vm_name --metadata snap_name

This procedure seems to work but only with VM with a single disk. If I try to create a snapshot for machines with 2 disks the procedure fails. For instance: if a VM has 2 disks vba and vdb and I try to run virsh snapshot-create-as --domain vm_name snap_name --diskspec vda,file=snapshot_folder/snap.qcow2 --disk-only --atomic --quiesce the command returns error: internal error: unable to execute QEMU command 'transaction': Could not create file: Permission denied Why? I tried with 2 --diskspec parameters, one for vda and one for vdb, but the result is the same.