Speed USB IMac 2020

iMac - Technical Specifications - Apple

Two Thunderbolt 3 (USB-C) ports with support for:

• DisplayPort
• Thunderbolt (up to 40Gb/s)
• USB 3.1 Gen 2 (up to 10Gb/s)
• Thunderbolt 2, HDMI, DVI, and VGA supported using adapters (sold separately)

of cause if you connect a mechanical external hd that will be the bottleneck as they max out around 100MB/sec

unless it's in an usb2 enclosure which would bottleneck it to around 40MB/sec

What external SSD do you have?

430MB/s sounds correct for a USB 3.0 connection.

What cable and/or adapter are you using to connect the External SSD to the Mac's USB-C port?

It sounds like you have a USB-C to USB-A cable or adapter in the mix that only supports USB 2.0 speed.

When I looked up your WD drive this is what it indicated:

"The WD_BLACK™ P50 Game Drive SSD also features a SuperSpeed USB 20GB/s (USB 3.2 Gen 2x2) interface...."

My recommendation is if you still can return the drive and buy one of these:

https://eshop.macsales.com/shop/owc-envoy-pro-ex-thunderbolt-3 if you want extremely fast speeds!

In addition to rkaufmann87's excellent advice, try contacting WD Support and see if they can suggest a different cable or solution.