I have a PR2100 running 2 x WD60EFAX (6TB) (RAID 1)
Disk 1 goes out every couple of weeks (starts blinking red) - so i decided to replace it
1- What are the steps to replace a faulty HDD without losing the data ? couldn’t find anything in the manual
2- i received a new 6 TB HDD but its WD60EFPX and not WD60EFAX (guessing WD60EFPX is the newer version) - is it OK to mix EFPX and EFAX together in the same PR2100 or they have to be exactly identical
You can simply remove the bad drive and replace it with the new one. Once the system sees it’s a new drive, it will start rebuilding the mirror. Your data is still accessible but response will be slower. I would not write any files (new or edit) until the rebuild finishes. How long that takes depends on how much stuff you have on it.
As far as the drives, the old one appears to be an WD Red (SMR) drive and the new one is a WD Red Plus (CMR). You can do a web search to find out the technical differences (SMR vs CMR). CMR is better for raid systems (IMO).
One thing to note, if one drive has gone bad, the other may also fail soon. You might consider getting another one. Once the mirror has rebuilt, you simply swap the other drive out with the new one and repeat the rebuild.