Disk Utility will not erase former MacBook HD

Hello,

I have swapped my MacBook 13 inch 120 gig HD for a 250 gig HD. I am running Snow Leopard. I used the cloning method with an external case. All went well.

I would now like to use the old HD as an external drive. However, Disk Utility erase options are greyed out as though I am booted from the old external disk and not the new internal. I have ejected the disk a few times. How can I work around this?

Do I need to find another mac on which to erase the disk? Will it even work or will that mac also see it as a bootable disk? I have erased old HDs post swap in the past this way but not with Snow Leopard.

Do I need third party software?

Thanks