For those who still have Dreamcasts, here is a simple guide to creating backups of your Phantasy Star Online saves. This method will work with any Dreamcast saves and is also useful for transferring data from one VMU to another.
The first thing you need to do is get a web browser for your Dreamcast that allows e-mailing of VMU data. One such browser is "Web Browser 2.0 with SegaNet," which shipped with many Dreamcasts. You will also need a POP3 e-mail account. Google's GMail provides such accounts. Set up the web browser to use the POP3 e-mail account according to the browser's instructions or manual.
Connect to the internet using your Dreamcast and use the browser to send an e-mail to your POP3 e-mail account. You can add VMU data to the e-mail as an attachment.
Now you can download the e-mail, or just the attachments if using a service like GMail, to your computer. This gives you a back up of your VMU data on your computer!
To transfer the VMU data back to the Dreamcast, send another e-mail to your POP3 e-mail account, this time from your computer, with the VMU data as an attachment. Then connect to the internet using your Dreamcast and retrieve the e-mail there. If you want to place the data on a different VMU, just swap the VMUs before retrieving the e-mail on the Dreamcast.
If you encounter problems saving the data to the VMU, check the file extensions. Some e-mail services may change the extensions.