Ran into an interesting situation on IRC today.
Solr has been installed as a service using the shell script install_solr_service.sh ... so it is running as an unprivileged user.
User is running "bin/solr create" as root. This causes permission problems, because the script creates the core's instanceDir with root ownership, then when Solr is instructed to actually create the core, it cannot create the dataDir.
Enhancement idea: When the install script is used, leave breadcrumbs somewhere so that the "create core" section of the main script can find it and su to the user specified during install.