To replace a failing drive is surely very important. To do it with a larger drive will not increase your usable space at first. You need to replace every drive in a vDev before it can auto-expand. You did not said how your 6 drives were configured, but I assume they are in a single Raid-Z vDev. In this case, you need to replace all of them before auto-expand can happen.
As for RAM, it is not related to drive errors or storage need. FreeNAS will never have too much RAM, so you can always put more.
How do I find our the config of drives? Also, I want to go to the latest version of FreeNAS. It has an 8Gb stick, so adding another is easy. Since this is for plex, do i need a cashe drive like a cheap SSD. That would involve adding a SATA card if I stick with 6 drives.
Also, can FreeNAS use 2.5GB NIC cards?
What you need most here is to define your needs and means :
--What budget do you have?: $500.00 Wanting 20TB usable
--How long do you wish to keep this NAS running?: 24/7
--How much you rely on that system to safeguard your data?: Used daily as a Plexserver
--If you do rely on it, how do you backup its content as of now?: No backup now, but building a backup server
--Is your backup a complete solution enforcing the 3 copies rule?: No, I dont have a location for a off site backup, but want 2
--How much storage do you use actually?: It grows constantly as I get more movies/TV shows. I also want to back up all computer data.
--How much data do you add every year? Hard to say, but at least 1-2 TB would be my guess.
--What do you expect to do with your storage for the next 5 years? This is files that I dont want to lose,so I will keep the drives up to date.
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Once you answered these, then you will have a better idea of what to do to ensure you reach your goals.