I am using the SD Card Copier accessory to make a micro-sd card back-up. The back-up card (in a micro-sd to USB adapter) is recognized by the copier as sdb, and I know that the copying is OK, because I can replace the OS micro-sd card by the back-up card, and the Pi boots successfully with everything in place.
On my previous Pi (model 3B) I could check the back-up card by simply removing it from the USB port (as it was unmounted) and re-plugging it so that I could inspect it via pcmanfm.
On the Pi 4 the re-plugged micro-sd back-up card is not recognized by the system and is impossible to mount or read. I can read it in the File Manager on my desktop PC, so there is presumably no problem with the micro-sd card itself.
Is there anything I can do to read the back-up card on the Pi 4?