attach vhd access denied - but works in computer management

I'm running windows server 2008 r2 sp1. When I attach a VHD as read only via Computer Management. It works fine.

However when I do it via disk part, example:

select vdisk "vdisk location"
attach vdisk readonly

I get an access denied error.

Whats the difference between attaching it via computer management and attaching it via diskpart?

I'm trying to automate the process of attaching VHDs, hence why I was trying disk part. Would there be an alternative way?