I think an expression of:
sum(//node[@class='disk']/size[@units='bytes']) div 1024 div 1024
would have worked for both of these nodes. However, I don't what a change like that might break.
I think an expression of:
sum( //node[ @class= 'disk'] /size[@ units=' bytes'] )
div 1024 div 1024
would have worked for both of these nodes. However, I don't what a change like that might break.