I use same iso file of Manjaro KDE installation for installing Manjaro on my old SanDisk 64Gb 3.0 Usb flash drive and installation done without problem. Now I bought new Samsung Plus Duo 128Gb drive and it is not listed during Manjaro installation when I should choose disk where to install the distro. With Manjaro KDE Partition Manager, I make my usb flash to be a gpt, and formatted it to different file formats. Nevertheless, Manjaro Installator cannot list my flash drive.
Depending on the USB device it announce itself as removable or non removable and I am pretty sure the Calamares installer is coded so it prevents the user from accidentally using the USB boot device as target for the installation.