KB ID 0001517
This sort of highlights my lack of experience with Azure! But I had to Google it, so if you are here, you did as well 🙂
There is currently a lease on the blob and no lease ID was specified in the request.
I had been uploading .vhd files and creating VMs from them, and was tidying up, so I wanted to delete anything that had been left behind.
All Services > Disks (or Disks (Classic)) > Select the appropriate disk > Delete.
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