HI ikmalj
If the Basic Disk storage unit is on a Flex instance, then unless you know what you were doing, the backups on the storage will probably disappear if you restart the instance (this is by design as an instance is a docker container that is rebuilt when it starts from a template). Basic disk storage units are not supported on a Flex instance - only MSDP and Advanced disk (the option is there to support BYO media servers). Storage pools are first created via the Flex console, then you configure this storage in NetBackup (instructions are contained in the various Flex instance and admin guides).
As for what you have asked - this is also not supported, you cannot import the catalog from a Flex master server into a BYO RHEL server, it wont work (the services on the RHEL machine are unlikely to start fully if at all). There are dependencies in the database specifying the master name and various path locations).
Let's go back a step - what are you trying to achieve with the RHEL server? From the answer to this, the forum can help guide you.
If you are looking to create a DR master, then maybe you should review the material on NetBackup AIR.
Cheers
David
David