I've had the same issue on a fork of Arch called Manjaro and posted to their forums without any solution. For brevity, I'm not gonna repeat what I've written there.
To sum up: I boot the computer from the USB - that is, the USB is the installation medium (yes, I have selected the network install and used Rufus in DD mode) - select any of the two install options, graphical or not, and get stuck on the detecting CD-ROM part. I have found out that this is because of the USB disappearing from the machine (I found this out by opening the terminal at this point, typing the command "blkid" and only seeing my target hard drive and no USB), and no unplugging and replugging helped so far (I have tried USB 3 and 2 ports, front and back of the desktop). This coincides with the Manjaro installation where it gets stuck on "device did not show up after 30 seconds", it too cannot find the USB during the installation. My motherboard can of course, after all, I started the installation somehow.
My computer specifications are:
- CPU: AMD Ryzen 3 2200G (4 core @ 3.5 GHz)
- GPU: AMD Radeon R9 380X
- RAM: 8 GB DDR4
- Motherboard: ASUS PRIME B450M-A