MKR1000 not detected on USB port (Mac)

Development Tools IDE 1.x
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Mac version of Arduino 1.8.19. -- Many times the MKR1000 is not in the port list.

I am comparing an older version of my program on a MKR1000 (let's call it M1), with a slightly later version of my program on a different MKR1000 (let's call it M2).

Note, this has nothing to do with the program. I've tried samples and swapping the code and I get the same problem of the MKR1000 not always being recognized.

Steps:

  1. I plug in M1, select the port, upload the older program, and monitor the results on the terminal.
  2. I swap the connector from M1 to M2, the port is still selected, upload the newer program, and monitor the results on the terminal. I take note of the differences in what the two program reported.

so far so good... but then...

  1. When I switch the connector back to M1, the MKR1000 is 'not always' listed in the port list! Just Serial Bluetooth... Sometimes I can put it back to M2 and it will show up in the list, sometimes not.

So I check the system information, and sure enough sometimes the MKR1000's show up and other times they don't. No matter what I do... Tried tons of combos: double click the reset to go into boot loader mode, hold reset a long time, hold reset while plugging in USB or pulling it out. pull the out the USB cable during boot loader mode and every combo... eventually the MKR1000 is recognized and I can move on.

THEN I GOT A CLUE... It seems like it's a timing thing. About ever 3 to 4 minutes, the USB list is flushed or something, because I started to notice that it would suddenly start working while I tried every combo I could think of. And what worked was almost always different and rarely the same two attempts would work.

A little more experimenting, and it became that clear the serial number of the MKR1000 was being associated with the USB port ID. Some kind of caching optimization to speed up initialization of devices... But if I waited a few minutes and left them unplugged.

So unless I can force the list to be flushed and the ports rescanned on demand I will have to flush the SCM or PRAM or whatever at hard boot... keeping in mind that simply rebooting/logging out didn't always seem to fix this.

So, anyone else see this? Is there a way to flush the USB cache?

Thanks!
Scott

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