The one in your web site is just created by ASP.Net by default when you
enable membership on it. Unfortunatley, it's mostly useless because there
are very few hosts that have SQL Server Express available in theor hosting
packages, which is required to attach an mdf file defined in a connection
string.
As for how it came into your SQL Server Express edition itself, I'm not
exactly sure. When I tried SQL Server Express a few ties (normally use full
version) it never did this. You may just be blessed though as a db that's
actually attached to SQL Server Express should be easier to get published to
a full SQL Server instance on your host.

Signature
Hope this helps,
Mark Fitzpatrick
Microsoft MVP - Expression
> I'm a newbie when it comes to SQL Server and am trying to understand why
> there is an aspnetdb.mdf created as a system database in SQL Server 2005
[quoted text clipped - 3 lines]
> Actually, I guess I understand why I get one in my App_Data folder of my
> web site, but what is the system one for?
Steven Cheng[MSFT] - 04 Feb 2008 04:04 GMT
Thanks for Mark's input
Hi Scott,
The aspnetdb in your web application App_Data folder is a feature of SQL
Express edition called "user instance" which can help you attach a flexible
mdf file with non-admin privilege.
#User Instances for Non-Administrators
http://msdn2.microsoft.com/en-us/library/ms143684.aspx
As for the aspnetdb in your SQL Express system database, I'm also a bit
surprised as Visual Studio or .NET won't install database into SQL EXPRESS
server instance by default. Is the box also accessible by any other members
on your side? There is a commandline utility in .net 2.0 that can help
create aspnetdb (all the sp and tables for ASP.NET provider services) in a
specified SQL instance:
#Herramienta Registro de SQL Server para ASP.NET (Aspnet_regsql.exe)
http://msdn2.microsoft.com/es-es/library/ms229862.aspx
You may check whether anyone else may have created the database.
Sincerely,
Steven Cheng
Microsoft MSDN Online Support Lead
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--------------------
>From: "Mark Fitzpatrick" <markfitz@fitzme.com>
>Subject: Re: aspnetdb.mdf
>Date: Fri, 1 Feb 2008 23:34:35 -0600
>The one in your web site is just created by ASP.Net by default when you
>enable membership on it. Unfortunatley, it's mostly useless because there
[quoted text clipped - 15 lines]
>> Actually, I guess I understand why I get one in my App_Data folder of my
>> web site, but what is the system one for?