'mount -a' works fine as a one-time action. But auto-mount of removable media reverts to settings that were in fstab at the last reboot.
How to make the OS actually reload fstab so auto-mounts use the new settings when media is connected?
Specific example seen with Raspbian (Debian) Stretch:
FAT-formatted SD card; configured fstab to auto-mount; rebooted; volume auto-mounts, but RO
Changed umask options in fstab; mount -a while media is connected, and volume is now RW
Unmount and re-insert the media; auto-mount works, but using the options in fstab from the last reboot, so volume is RO
Reboot; OS loads updated fstab; auto-mount works when media is connected, and volume is RW - how to get this effect without a reboot?
FWIW, the (updated) fstab syntax was:
/dev/sdb1 /Volumes/boot vfat rw,user,exec,nofail,umask=0000 0 0