Well, I have discovered a/the solution. As described, if I open Computer and try to open the second HD (sdb1) as Adminstrator then I get a refusal message - and no Permissions are listed. However, if I just open the drive but then go back up a level (using the up icon arrow) it is then possible to open as Administrator and, having done so, have full control to change permissions, open new folders, etc.
The foregoing works without having the drive mounted on startup so the fstab file can be left uinchanged.
Anyway, thanks for responses.