I picked up a cheap Arduino Uno clone board off eBay, for some projects I am working on (to be featured here, once I have a bit more to show).
There’s lots of these on eBay, this particular one is listed as “Hotsell UNO R3 ATmega328P Development Board With Boot Loader For Arduino UNO BG”, it was $3.45 shipped, not a bad price. I see that since my purchase, the seller has raised the price to $4.76.
It arrived fairly quickly, about 3 weeks, typical for stuff from China. I plugged it in, fired up the Arduino IDE and… nothing. Didn’t show up. Checked the list of USB devices via System Information, and it did appear, with vendor ID 0x1a86 and product ID 0x7523.
Some web searches showed up lots of hits for this, particularly dealing with cheap Chinese Arduino boards. Apparently they use a CH34x chipset based USB interface chip, which your computer often does not have drivers for. Some more searching turn up this page: https://0xcf.com/2015/03/13/chinese-arduinos-with-ch340-ch341-serial-usb-chip-on-os-x-yosemite/
With a download link for a Mac OS X driver: https://0xcf.com/download/ch341ser-mac/?wpdmdl=122
They have Windows drivers as well.
I ran the driver installer and… nothing. Still does not show up in the Arduino IDE, nor in the list of devices. I poked around a bit, and noticed the installer put the kext into /Library/Extensions/ and not /System/Library/Extensions which I know can sometimes cause problems. So I moved it to /System/Library/Extensions via:
sudo cp -R /Library/Extensions/usbserial.kext /System/Library/Extensions/usbserial.kext
And sure enough it now appears in /dev:
/dev/cu.wchusbserial14364440
And appears in the Arduino IDE, and works fine. Success!
Writing as a long-established Mac user, but not experienced at delving into the inner workings, I wondered why you did not just move the driver manually. I have just tried to, but was not allowed to (“system library cannot be modified” or similar wording). Is this why you did it with Terminal? I have never dared to use Terminal, often seeing warnings that it can ruin your Mac.