This may be a question more generic than for just the DGL4500, but is there a way to attach a USB hard drive and then access it via the internet when not connected to the router's network? Or is it only accessible when on the network? If not, would you (or could you?) get a drive that attaches to the network via ethernet/wifi and then have remote access?
There are a number of stand-alone NAS devices that do what I think you're looking for...
Make sure you check the feature sets of the devices you consider. I use a 1TB Western Digital My Book World Edition which attaches to the network via a Gigabit Ethernet connection, but it relies on the MioNet service to provide HTTP access outside of your local network.
If you have a server machine on your network, even one running workstation versions of XP, Vista, or 7 you could attach the drive to that machine, and then share it out to the Internet. That machine would need to run 24/7 or as needed to be able to access the drive. But it can be done.
Hope that helps,
Hank