This is a spin-off of another topic. New readers can safely start here...
Looks like those folks who urged me to switch my backup HDD to JFS from FAT32 were right. In summary: I have been updating important files, some moderately large (see below), which I back up to two USB-3A drives, one SSM (around 32GB, JFS), the other HDD (2TB, FAT32). Various folk have helped with suggestions on switching the HDD to JFS. Experience and experiments have indicated that is pretty much my only course -- if my system dies, I'll have to wait until it's fixed/replaced and hope both JFS backups stay safe meanwhile.
The evidence: seven (7) files have been involved, daily, growing as I add data to them. Sizes are roughly: 95KB, 483KB, 159KB, 23KB, 27KB and 32KB. When I drag-and-drop-copy them from my system (JFS) to the JFS SSM drive, it goes smoothly and quite fast (if we keep USB's sluggishness in mind). If I D&DC to the FAT32 HDD drive, the five (5) smaller files are shown on the tally as copying over but the two biggest lead to a freeze of the tally and system, and a need for CAD+R. But sometimes the biggest files do seem to get copied over.
Does any of this make sense? ISTR someone saying to partition the 2TB drive to prevent XCOPY (and, I assume, similar mechanisms invoked with D&DC) from exhausting buffer space, IIRC.
(Sidebar, noted during these experiments:) Ejecting an USB MSD did not automatically close some folders connected to that MSD. But since then I have used the new CHEKINI, which found some rubbish and threw it out, so the rubbish may have been the cause. Later, the USB "Eject" function refused to eject until I had closed dependent folders that happened to be open, saying the USB device was busy.