Thank you Melophage ! I find the UNIX commands less confusing now, almost enjoyable ! :-))
But I tried with 3 different USB keys where I just created an empty <trial> directory.... And each time I got : "cp: /Volumes/MobileBackups/trial: Read-only file system" ! Two of them are formatted "MS-DOS (FAT 32)" and "(FAT 16) " respectively. Third one I didn't check but surely also FAT 32. What am I to do ?? [Shocked] [Laugh]
I finally was able to copy my critical file from my crippled Mac to my USB drive, while in single-user mode !!!
But it took a lot of work... First we found that the whole file system is read-only by default and must be changed to read-write. Then I discovered that my USB drive was in fact not visible/mounted, and that the MobileBackups Directory had been created by the system within Volumes, on my HD... Finally we found a recipe on how to do it from the start :
I highly recommend it !