After a few hours of Googling and testing things I finally solved this problem so I thought I’d write the solution down both for my own and other’s benefit.

What I did to get the error:
I had just done some changes to one of my web sites using Visual Studio 2005 and ASP.NET 2.0 with VB.NET. I have previously used MySQL as a database for this site but thought I would test using SQL Server 2005 Express Edition for a new feature I just added.
As usual I did Build – Publish Web Site and then copied and pasted the published directory to my web server. When browsing any of the web pages that use the SQL Server 2005 database I get this error message: Failed to update database “XXX.MDF” because the database is read-only.

Solution:
From what I found on Google this seems to be a confirmed bug in the beta releases of Visual Studio but should be fixed in the released version, but obviously it is not. There seems to be many ways and suggestions on how to fix this, some very complicated.

This worked for me:
1) Make sure App_Data directory or any contained files does not have file system attribute Read-only set. (I had some files marked as Read-only)
2) Give user ASPNET and NETWORK SERVICE Modify control over the App_Data directory. (I had to add both these)
3) Run IISRESET to restart IIS to refresh its permissions.

Main part of this solution was found in this MSDN forum thread, specifically posts by Thongtap and Justin. Thanks guys!