I decided to replace the 500GB drives in my TSLive with 1500GB ones, after one of them failed.
Unfortunately I rebuilt the Raid:5 array (yes, once for each new drive - took almost 2 whole days

) before reading the Wiki
here.
The article
here talks about increasing the size, but it also warns that one has to do some other steps before doing so.
Steps which I obviously missed, when I used the Buffalo web interface to rebuild the array.
What exactly must I do now to grow the array to the full size please?
Thanks for reading!