That's perfectly normal. If it's not working, the problem is on the Maxtor drive.
In SMB terms, there's no such thing as a username without a "domain". In a corporate environment, the user object resides within Active Directory (AD) and the fully-qualified username will be something like "HEADOFFICE\PointyHairedBoss". Because your home machine is presumably not part of an AD domain, you're logging in with a local account and the domain name that's forwarded is the name of your PC - that's where the account database which contains the user definition resides.
When receiving an access request, a machine is meant to check whether the domain name matches its own, a "trusted" domain (in which case the authentication request is forwarded), or whether it's unknown. The Maxtor NAS presumably treats your machine as an unknown domain, so it's supposed to ignore the unknown domain label and attempt auth against its own account with the same name - if it exists. That's called pass-through authentication, and if the Maxtor NAS isn't doing it, that's a problem.