Disc does not show

What shows up in the Volume Settings page (under Volumes in the menu bar on the left) in Frontview?

What version of RAIDiator are you running?

If the disk is accepted, the system will accept a disk equal to or larger than the smallest disk in the array. Since you have two 750GB disks, a good 2TB disk should be accepted. However, the WD20EARX is a 4K sector disk as compared to the 750GB disks previously in the array which are 512K sector drives. In order to support 4K sector drives (most of the new ones are) you need to perform a factory default on RAIDiator 4.1.7 or later.To do this you would have to update your current and complete backup, update RAIDiator and then do a factory default. Restore the data from the backup and then hot add the 2TB.

The Volume Settings page is empty, actually it's the USB tab that shows up, empty.

The cersion is RAIDiator™ v3.01c1-p6 [1.00a034]. Does that mean that I should still perform a factory default? And/or update the RAIDiator?

Both. Backup your data if you haven't already, update to the latest firmware and after verifying the firmware update was successful (should see redesigned web-interface with NetGear not Infrant shown in Frontview: http://www.readynas.com/simulators/frontview/) then do a factory default. Also note that with RAIDiator 4.x the default password has changed: http://www.readynas.com/kb/faq/boot/what_is_the_frontview_default_login_and_password_after_a_factory_default

Your currently installed firmware was released over 5 years ago back in December 2006!: http://www.readynas.com/forum/viewtopic.php?f=1&t=7836

Thank you very much. That worked out just fine :)

However the memory did not increas as much as I had hoped. It is now only 997 GB (before it was 716 GB). After all I added a 2 TB disk. Is this how it is supposed to be?

Ok, you are on an NV+ (v1) so what disks are installed (make and model)?