Yes for some reason we encountered all the problems when creating the backup on Windows 7 that I was excepting to meet applying the restore on Windows 8. I suppose that may still arise when you eventually decide to back up the database on Windows 8.
To that end, and for anyone who stumbles on this thread while looking for an answer a quick overview of what happened.
1. Install Microsoft SQL Server Management Studio Express 2005
In order to easily address the "server principal X is not able to access the database" error we need Microsoft SQL Server Management Studio Express 2005 (also know as SSMSE). If you don't already have SSMSE you can download it from this page. It comes in both 32-bit and 64-bit flavours so ensure you download the right one. If you don't know which version you can refer to this page.
2. Fix the "installer has encountered an unexpected" error
The SSMSE installer (.msi package) has to be run with administrator privileges. If your aren't logged in as an administrator the installation will fail with the following 29506 error: "The installer has encountered an unexpected error installing this package.This may indicate a problem with this package.The error code is 29506". To resolve this you can use an administrator command prompt to run the installer.
Open an administrator command prompt and enter the the full file path to and name of the installer package. If you don't know now to open an administrator command prompt see this page. If the path has any spaces in it you must wrap the whole path in double quotes. You must remember to include the .msi extension (which may not be visible if Windows is hiding common extensions). For example:
C:\Windows\system32>"C:\Users\My Username\Downloads\SQLServer2005_SSMSEE_x64.msi"
3. Fix the "server principal X is not able to access the database" error
The backup and restore functions have to be run using the sysadmin fixed server role and the db_owner and db_backupoperator fixed database roles. If your windows account isn't a member of one of these roles the operation will fail with the following error: "The server principal "ComputerName\UserName" is not able to access the database "bw_dragonage_content" under the current security context". To resolve this you can make assign yourself the db_owner role for the Toolset's database.
Start SMSSE and when the "Connect to Server" window appears ensure it is configured as follows and then click connect:
Server name: .\BWDATOOLSET
Authentication: Windows Authentication
Once you are connected locate the Security folder in the Object Explorer. Expand it and right-click on the Logins folder and select the New Login ... menu option. When the Login - New window opens (at the General page) enter the Login name as it appeared in the error message, i.e. ComputerName\UserName.
Alternatively click the Search... button. When the Select User or Group window opens enter your user name in the Enter the object name to select (examples) text box and click the Check Names button. This should complete your user name. If it does not you can click on the Advanced ... button for more options. Click the Okay button.
Leave all the other settings on that page at their default settings and select the User Mapping page. In the Users mapped to this login list check the check box beside the bw_dragonage_content entry. Your user name will be added to the entry in the User column and the Database role membership for: bw_dragonage_content list. In that list check the checkbox next to db_owner.
Click Okay.
Truth be told this step took a couple of attempts. The first time we tried it, we couldn't access the User Mapping page without getting a 916 error. However after closing SMSSE down, reopening it, expanding the Security and Logins folders and reopening the new login we were able to set the mappings correctly.
4. Fix the "Cannot open backup device" error
Creating or restoring a backup takes a minute or two (the official resources alone are 687MB). If you double click on the batch file and it completes instantly then it probably failed. You can confirm this by opening the opening the log.txt file created in the Backup_Restore folder. If the operation terminated with something like following error (there are a few variations) then you need to give the SQL Server service access to the backups folder:
Msg 3201, Level 16, State 1, Line 1
Cannot open backup device 'C:\Backups\bw_dragon_content_001.bak'. Operating system error 5(error not found).
Open the Services MMC (Microsoft Management Console) snap-in. If you don't know how to do this refer to this page. Scroll down to the SQL Server (BWDATOOLSET) entry and double-click on it (or right-click and select the Properties menu option). When the Properties window opens select the Log On page and make a note of the name in the This account text box (it should be Network Service). Close the Properties window and Services snap-in.
Navigate to your C: drive, right-click on the Backups folder and select the Properties menu option. When the Properties window opens select the Security page and click the Edit button. When the Permissions window opens click the Add ... button. When the Select User or Group window opens enter the account you noted above (for example Network Service) in the Enter the object name to select (examples) text box and click the Check Names button. This should complete your user name. If it does not you can click on the Advanced ... button for more options. Click the Okay button to return to the Permissions window. In the Group or user names list select the new entry you created, then in the Permissions for [user name] list check the checkbox in the Allow column for the Modify and Write entries. Click the Okay button to return the Properties window and click the Okay button to update the folder.