In order to Boot (DOS/Windoze) - you need a Primary Partition - it can only have 1 drive letter. An Extended Partition is the other type that DOS/Windoze recognises - it can have multiple 'partitions' inside it - each of which will be given a separate drive letter.

The disadvantage of having no primary partition on your second drive is that if the first drive goes belly up, you can;t get running quickly by using the second drive without mucking about with it to give it a primary partition which you can make bootable.

For me the 'disadvantage' of having D drive on the second had drive is greatly outweighed by the ease of future potential 'disaster proofing'.

Robin