If a preferences file is specified with the --prefsfile parameter, slimserver
refuses to write to the specified file for the wrong reason. The code in
Slim/Utils/Prefs.pm contains the following test:

$canWrite = (-e prefsFile() && -w prefsFile()) || (-w preferencesPath());

When running with --prefsfile /etc/slimserver/slimserver.pref,
prefsFile() is '/etc/slimserver/slimserver.pref', but
preferencesPath() is '/root'. This means that the code checks to see
if /etc/slimserver/slimserver.pref exists, and if it doesn't exist it
checks to see if it can write to /root, which it can't and thus an
error-message is printed:

Cannot write to preferences file
/etc/slimserver/slimserver.pref, any changes made will not be preserved
for the next startup of the server

The correct check is to see if it can write to /etc/slimserver, but I don't know
if preferencesPath() is wrong or if the check -w preferencesPath() is wrong.