Install directory is the place where the slimserver program files and data files are stored. The installdir can only be changed when slimserver is stopped and when the service is being enabled. If /usr/local/* is used the slimserver data will be lost on reboot since root-filesystem in unRaid is ram-based. Move to /mnt/ to persist slimserver library data across reboots
If slimserver is installed on a disk, that disk and the parity disk might not spin down (though slimserver 7.7.1 does not appear to have that problem). Install on cache disk, or usb flash (preferably extra non-unraid-'licensed' usb-stick, to avoid premature death to licensed usb flash by constant writes), or /usr/local/slimserver (ram based, thus non persistent on reboots).