It's a not a bug. It's by design.
Basically, the phone takes the on-board storage and the card and creates a single storage pool or "drive". So if you have 8 GB on your phone and a 16 GB card, your phone sees it as one 24 GB storage pool and it just saves stuff either on the phone or the card without any specific care as to what goes where.
If you remove the card, you won't be able to boot up the phone. Also, the card is digitally and securely attached to the phone, so you won't be able to view photos or music stored on it using your PC.
In Windows Phone 8, phones with micro SD slots treat the card as separate storage location and it can be added or removed at-will. It can only really store photos and music though, not apps.
11-07-2012 03:18 PM