It appears to me that each Arduino has (should have?) a unique /dev/tty...USB... number assigned. Why this isn't explained anywhere (that I can find) is a mystery. When I plug my problem Nano into a USB port on my iMac no /dev/tty... for it appears. It used to work. Have I fried it? Can this be fixed?
This is probably a duplicate question and duplicate answer, but ...
Flip the board over, look at the bottom. If it says "FTDI" then this answer doesn't apply. But many cheap clones have moved to the CH340G chip for USB serial communication, it's a tenth of the cost with a tenth of the (unused) features.
Or, skip the USB altogether and search for "ISP" on Tindie and grab a USBASP or TinyISP programmer.