SOLVED: How To Connect an IDE VHDX to a Generation 2 HyperV Virtual Machines

The short answer is you can’t:

…the emulated IDE controller is no longer present in a generation 2 Virtual Machine (VM)…
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Many people think this is because the throughput on virtual SCSI controller is faster than the virtual IDE controller but that has not been true for many years.  The reason Microsoft removed the IDE controller is because it takes alot of overhead:

(with IDE) …during a guest operating system boot, each I/O involves 6 (9 if 48-bit LBA) round trips through the hypervisor, the parent partition kernel mode, parent partition user mode and device emulation inside the worker process to complete. This is actually quite a significant overhead in terms of compute cycles, and also causes the guest VMs to be paused during this emulation path…

…With native support for VMBus in our UEFI firmware in a generation 2 VM, no emulation is necessary to boot a VM from a VHDX or ISO attached to a virtual SCSI controller. Hence initial boot performance is far more efficient…
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The real advantage to Generation 2 virtual SCSI over virtual IDE is that many disk operations including expanding the disk and added additional disk can be completed with the Virtual Machine running.