Gnome Disks is not detecting all external hard disks connected to PC. If more
than one external hard disks are connected to PC then Gnome Disks will only
detect one external hard disks leaving all other hard disks. If detected
external hard disk is safely removed from PC, only then it will detect other
external hard disk that was connected with PC but still leaving the rest of
external hard disks. But files app is detecting all external hard disks.

For example: If three external hard disks are connected with PC then only one
of them will be detected by gnome disks app, leaving other two while they are
still connected with PC. If detected external hard disk is safely removed from
PC then it will detect second external hard disk still leaving the third one.
And if second is removed from PC only then it will detect third one. Hence all
connected external hard disks are not detected in parallel or at once.