Based on your description, the Intel® SSD 750 is working correctly. This type of behavior seems to be related to the way you motherboard works, here are few things for you to consider about this:
- Some motherboards map the PCIe NVMe SSD's to the Storage Chipset, and may even disable a SATA port when the NVMe drive is connected. You may to try using a different SATA port for the 3.5" Hard Drive if possible.
- We advise you to contact the Computer Manufacturer Support Websites, they will be able to provide details on how your computer handles PCIe SSD's with NVMe technology, and if they have any recommendations to make all your drives work at the same time.
- Also, check with HP support to make sure you are using the most recent BIOS, and any additional configuration applicable to your system.