New MacBook Pro, using an FTDI USB serial cable (used to program ESP32 chips).
The error was "Resource busy".
The problem is that googling this gets a lot of people saying they have this issue and a lot of answers which didn't help me - it seems many people do this to themselves by having the device open from some other app. Using lsof I confirmed it was not open by something else. Some pages even suggested update to python, but I confirmed that even a simple cat of /dev/cu.usbserial-A... said the same, so not python. Someone even suggested it was not a genuine FTDI chip, so I ordered a cable from RS, and the same issue persisted.
After a lot of googling, it seems to me that there must be some sort of bug in Catalina. It looks like FTDI USB serial should work on a Mac without needing custom drivers, but it does not work. It is recognised and the /dev/ created as you would expect, but shows the resource busy error.
Importantly there was no pop-up or prompt or error of any sort when connecting the lead, it simply appears in /dev/ as you would expect. No clue that there is a problem with drivers.
So, what did I do?
- Install the VCP drivers from FTDI (2.4.2), it is a DMG you open and install. This all went swimmingly but did not help even after a reboot, and not obvious clue as to why.
- I used the About This Mac, System Report, Software, Extensions, and saw FTDIUSBSerialDriver 2.4.2 listed, Notarised:Yes, Loaded: No, which was a clue.
- Going to System Preferences, Security and Privacy, it showed text "Some system software was blocked from loading". However, this was subtle as the adjacent "Allow" button was not active. It was not immediately obvious but I had to "unlock" the window, and then I could click Allow, and select FTDI to allow the driver.