Replacement of 5 year old drives with new ones on a Terastation (best way)

[garyhgaryh]

I have a Terastation TS5400D I set up in 2017.  Last month we had a heatwave in the CA Bay Area.  Temps hit 112F in Danville and that night my Terastation's alarm went off (the NAS is not in an air conditioned room).  I logged in and noticed my NAS running in degraded mode.  Two drives failed (drive 1 and 4 and I'm running RAID5).  I tried to browse my filesystem but as I was going into the directories, the Terastation stopped responding, but webserver is still working so I powered down and restarted.  Again I was able to get into the filesystem, but the filesystem disappeared again.  Both times the array went offline.  I let it sit about an hour then powered it down to figure my next course of action.

A month later (today), I turned my Terastation back on again, but it was unresponsive.  Webserver does not respond but NAS responds to pings.  Powered cycled again (but this time by holding the power button).  I finally was able to get to the web server.   Again Drives 1 and 4 are at fault.  Drive one is so bad the NAS can't even determine the model of the drive.  Drive 4 shows the drive info and fault status.  To my surprise I was able to browse the filesystem without the NAS becoming unresponsive or taking the array offline.  I was able to manually copy all my files to a usb drive (took about four hours).  I was able to do this probably because my filesystem was less than 4% full after 5 years.

I bought new drives.  Whats the best way to replace the drives on the NAS? I want the user and directory structure and everything else I configured to be intact.  Can I run some sort of backup, remove all the drives, replace it with my new drives, and restore? I never done this before so this might be an easy task (apologies for not knowing).  I also want to select a RAID version (RAID6?) that will handle the loss of two drives without loss of data.  I'm very luck to be able to get all my data back with two drive failure running on RAID5, but I believe that was due to the small amount of data I had on the NAS.

Thanks in advance for your help!
Gary