It seems like something is wrong with the permissions on your external drive. By default, Windows XP runs the user account as Administrator, where as Vista and Windows 7 runs the user account with limited access, using UAC to control administrative access. As a result, the external hard disk may have its permissions set up for read/write access to administrators, but read-only to non-administrator accounts.
You can try the following steps:
- Go into "Computer", right-click the external drive and click 'Properties'.
- Go into the Security tab and click 'Edit'.
- Give full control to the "Everyone" user group and click 'OK'.
- If it requests, allow this to take effect in subfolders.
I'm not sure if Windows 7 Home Premium has the Security tab (for step 2.)