I don't think this should be a string:
```
➜  govuk_offsitebackups-puppet git:(vagrant-adddiscs) ✗ vagrant reload obnode0
[obnode0] Attempting graceful shutdown of VM...
[obnode0] Clearing any previously set forwarded ports...
[obnode0] Clearing any previously set network interfaces...
[obnode0] Preparing network interfaces based on configuration...
[obnode0] Forwarding ports...
[obnode0] -- 22 => 2222 (adapter 1)
[obnode0] Running 'pre-boot' VM customizations...
A customization command failed:

["createhd", "--filename", "/tmp/obnode_extradisc", "--size", "512 * 1024", "--format", "vdi"]

The following error was experienced:

#<Vagrant::Errors::VBoxManageError: There was an error while executing `VBoxManage`, a CLI used by Vagrant
for controlling VirtualBox. The command and stderr is shown below.

Command: ["createhd", "--filename", "/tmp/obnode_extradisc", "--size", "512 * 1024", "--format", "vdi"]

Stderr: Oracle VM VirtualBox Command Line Management Interface Version 4.2.6
(C) 2005-2012 Oracle Corporation
All rights reserved.

Usage:

VBoxManage createhd         --filename <filename>
                            [--size <megabytes>|--sizebyte <bytes>]
                            [--diffparent <uuid>|<filename>
                            [--format VDI|VMDK|VHD] (default: VDI)
                            [--variant Standard,Fixed,Split2G,Stream,ESX]


Syntax error: --size: RTGetOpt: Command line option has argument with bad format.
>

Please fix this customization and try again.
```