Disk Utility: "Waiting for partitions to activate"

My wife's Mid-2012 MacBook Pro had a failed internal HDD (it was the boot drive), so I created a Sierra USB boot with a view to running disk utility on a new HDD and re-installing Sierra. Good plan.

Disk utility progresses slowly:

1. Unmounting disk

2. Creating the partition map

3. Waiting for partitions to activate

Now been on stage 3 for 20minutes. No progress on the progress bar. I don't want to interrupt it unless it really isn't going to get anywhere. I've searched but not found anything particularly helpful about 'Waiting for partitions to activate'

Any ideas?