Entity Framework 6 GUID as primary key: Cannot insert the value NULL into column ‘Id’, table ‘FileStore’; column does not allow nulls

In addition to adding these attributes to your Id column:

[Key]
[DatabaseGenerated(DatabaseGeneratedOption.Identity)]
public Guid Id { get; set; }

in your migration you should change your CreateTable to add the defaultValueSQL property to your column i.e.:

Id = c.Guid(nullable: false, identity: true, defaultValueSql: "newsequentialid()"),

This will prevent you from having to manually touch your database which, as you pointed out in the comments, is something you want to avoid with Code First.