how many m.2 slots does this motherboard have? how many hard drive ports?

The motherboard has 2 M.2 slots and 4 SATA 6Gb/s hard drive ports.

How many memory slots does the MSI B550M PRO-VDH motherboard have and what is the maximum memory it supports?

The MSI B550M PRO-VDH motherboard has 4 memory slots and supports a maximum memory of 128GB.

The MSI B550M PRO-VDH motherboard uses the AM4 CPU socket.

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MSI B550M PRO-VDH Micro-ATX Motherboard - AMD AM4, Ryzen 5000/4000/3000 Ready, PCIe 4.0, Dual M.2, USB 3.2 Gen 1, 2.5G LAN, Realtek Audio

PCI-E Steel Armor: Protecting VGA cards against bending and EMI
Mystic Light Extension: Control additional RGB or Rainbow strip added to your system with Dragon Center.
M.2 Shield FROZR: Strengthened built-in M.2 thermal solution. Keeps M.2 SSDs safe while preventing throttling, making them run faster.
Lightning Gen4 Solution: The latest Gen4 PCI-E and M.2 solution with up to 64GB/s bandwidth for maximum transfer speed.
Audio Boost: Reward your ears with studio grade sound quality for the most
immersive audio experience.
Core Boost: With premium layout and optimized power design to support
more cores and provide better performance.
DDR4 Boost: Advanced technology to deliver pure data signals for the best
gaming performance and stability.
2oz Thickened Copper PCB: An enhanced PCB design improves heat dissipation and performance reliability.