My Canon MP560 - and I believe most other similar Canon printer/scanner devices - has three colour cartridges (C,M,Y) and a matching-size 521 BK black one. It also has a second, larger 520 PGBK black ink tank.

I print mostly text documents (black text with occasional limited colour elements). So the small black tank is used much more quickly than the colour ones - and when it is empty the printer shows dire warnings about not continuing until a new tank is fitted (even though the large black tank and the three colour ones are still full).

Why won't the printer switch to using black ink from the larger PGBK tank? How can I control, or manage, which black ink cartridge is used and when? Are the black inks different? I have heard it said that the small black ink, like the colour cartridges, is 'photo quality' while the larger PGBK ink is intended for text. If that is so, why is my text-printing using up the small "photo-ink" black cartridge?

I realise that one route may be to switch to "grayscale" printing. I presume - but cannot easily verify - that this would draw ink from the larger black tank. But to do this would prevent the very few coloured bits (eg in the letterheading) from appearing as they should, in colour.

Any advice or explanation would be appreciated.