Oh boy. Have done a search here prior to posting and didnt find anything that answered my question so…
I run Manjaro Stable and Testing on different drives on the same desktop, bare metal,UEFI. On testing tonight when installing the 510 kernel I ended up fragging GRUB-it was a somewhat involved change necessitating upgrading NVidia 450 to 455 to get the 510 kernel to install with the NVidia package (before my attempt I was running 450 on 59-and the install program would not load the NVidia package). Chroot didnt fix things, what did was reinstalling Stable on its drive,SDA- it re created the FAT32 partition, re installed GRUB, found testing on the other drive and all is good.
When I update either system, GRUB gets updated if a kernel is included. How does the first OS -in this case Testing-on SDB-know that GRUB is now on the FAT32 partition at the start of SDA? what needs to be done to make updates not a pain and make the entire system unbootable the way I did earlier? I have run this way for a while and so far have had no problems but earlier when things were broken I found 2 GRUB.cfg files on the FAT32 drive before reinstalling Stable on SDA…this seems like its probably a stupid question but my history here to fore has been running Windoze and Manjaro dual boot on the same system. This is the first time I have run 2 Manjaro installs on the same system(sans Windoze). If I now do an update to Testing on SDB, should it find GRUB on SDA and update that? At this point both Stable and Testing are bootable and running but with an update that requires a GRUB update, then what?