Trying to delete approx 8m of a 21million collection, its taking ages & appears to be very slow.

I am running a js file locally on server (mongo localhost:27018 remove.js) that contains:
collection.remove({'dts':{'$gte':ISODate('2012-06-21T00:00:00Z')}});

I only have two indexes.
one on _id.
one on above date field & id.

Disk I/O is maxxed out at 100%, please see attached iostat file.

I have an MMS account, please let me know what further details you need if you can look at it, the server is currently running the delete operation.

I also have another shard that I am also doing a delete on, but instead trying out with
var c = collection.find({'dts':{'$gte':ISODate('2012-06-21T00:00:00Z')}},

);
and
c.forEach(function (id) { cursor.remove({_id: id._id});})

It's also maxxing out I/O at 100% with a very slow return on deletes/sec.

Is there a recommended way to delete a large quantity of items?
Am I doing something blatantly wrong?