Good evening, Luc de Hemptinne,
I supposed your drives icons have a little lock, and that,
with Get Info, Privileges and Permissions are "Custom", isn’t it ?
Simply try (with
guaranteed success)
BatChmod by Renaud Boisjoly
(a frog eater like me),
or
FileXaminer by (I don't remember,
it's said on BatChmod site).
These softwares solve
ANY privileges and permissions issues,
and *even more*, but *be careful*.
They
both are
free, but you should give them $10,
as I did myself, when I solved my *Disks Locked* problem
(Have a look by searching "disks locked" in discussions.apple,
or search "Eww" or "Olivier Herrbach").
If you want something more expensive (is it reassuring ?),
MacPilot (Koingo), *TinkerTool System* (Marcel Brezinc),
iRepair,
Cocktail and many other softwares
(I can tell you the names by searching)
have dedicated sections to solve these …
let’s say, ennoying authorizations issues.
There's absolutely *no need* to go into Terminal.
I hope having helped you ?
Best regards,
Vialatte