You’re welcome but it’s a shame it didn’t fix your problem. I’ll address your specific questions and then come back to the problem.
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One of the advantages of Drobo is that it allows you to mix and match drive models, capacities, spindle speeds and manufacturers. There are some type of drive that shouldn’t be used, including hybrid SSHDs and the large capacity Archive drives that use Shingled Magnetic Recording technology, but the WD “colour” series are all compatible (see here and here for the official word from Drobo).
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Yes, indeed. That’s exactly the process. Ensure that all drive light are green. No need to power down, just pop out any one of the drives and replace it with a new one. Wait for the Drobo to finish rebuilding the array, during which the lights will flash yellow/green or amber/green. Once they are all green again you can replace another drive, which you’ll need to do in order to see any capacity gain.
I haven’t needed Drobo support yet and I do have a two year warranty but I’ve read many posts here from customers who feel unsupported, which is why I (any many others) try to help if we can.
So, back to your problem. I suppose both drives could have been damaged in transit, but it’s not very likely. Can you test them in a PC? A USB docking station or external case would do to check if they work, but to do anything more sophisticated you’d probably need to plug them directly into a SATA controller.
If the drives are ok then the problem must be with the Drobo but, since it was working happily until you tried to upgrade your capacity, it might have suffered some glitch rather than actual damage. So there are a few things you can try before writing it off.
First of all, it would be useful to know the current state of your disk pack. A four-slot Drobo can work safely with only three drives if you have sufficient capacity across those three drives to hold your data and the parity. So, with your three remaining 1 TB drives, you can safely store about 1.81 TB, according to the Capacity Calculator, fully protected. Therefore, if you have less than that amount stored on your Drobo my recommendation would be to leave it alone, with three drives and an empty slot, until it has finished its data protection routine. My idea is to get your data safe, which is the first priority, and my reasoning goes like this: you can’t make it recognise a drive in the fourth slot so use the remaining three drives as best you can right now, with a view to addressing the actual problem once your data is fully protected.
If you have more data than can be safely stored with protection across three drives, then you have a choice. Either copy off some files to somewhere else and delete them (hopefully temporarily) from the Drobo in order to make room for the parity data, or proceed with your data still present but unprotected against further failure. The latter option is risky and you may lose everything.
Once you’ve got three green lights or if you’ve decided to risk it, shut down the Drobo cleanly. Use the Dashboard if possible, or the power button if necessary. Take out all three drives and put them safely to one side. Now fix the Drobo! (*) Once it’s fixed and powered down you can reinsert all of the drives from your disk pack (they don’t even need to be in the same slots as before) and power it up and continue where you left off (i.e. insert a new drive into the empty slot).
And that’s all there is to it. Of course, there is one snag: I said “Now fix the Drobo!” Ok, so at that point your disk pack is safe to one side, hopefully fully protected but possibly not. You can now do some testing of the chassis and those two new-but-maybe-bad drives you have. At this point I invite people more familiar with the four-bay Drobos to contribute their experience as I have only a 5N, though most of the principles are the same.
Since it’s now powered down, it’s worth disconnecting all the cables and leaving it for ten minutes or so. Then re-connect the cables and power it up. Wait for the Dashboard to find it. Once it’s powered up and settled down it should invite you to insert a drive so give it one of those new drives in the top slot. Does it recognise it? If it doesn’t then try the other. If either is not recognised then it really is a dead drive. If the first drive is recognised then add the second into the second slot. That way you can sort good from bad drives. If one or both drives is shown to be good try moving it to the third slot. Is it recognised? Now try it in the fourth slot. If at least one drive works in all four slots then your Drobo is good - it’s fixed! Continue after the (*).
If neither drive is recognised, you were very unlucky but as compensation for your bad luck in purchasing drives your Drobo is quite possibly ok. You really need to get those drives replaced so that you can test all four bays. If all four work, congratulations, it’s fixed! Continue after the (*).
If either or both drives prove to be good but not recognised in one of the slots then the connectors at the back of that slot may be physically damaged, though they are fairly robust. Make sure the drive sits fully in its slot - it should click in and be held by the latch. If it isn’t mechanically damaged it would be worth trying a pinhole reset of the chassis. If this works, it’s fixed! Continue after the (*).
There are a lot of things to try. I’ve tried to lay out logical steps for you to follow. If you get to this point and it still isn’t fixed, then report back and maybe someone has something else to suggest. Even with one bay not working, you can still use it with three drives. I hope this helps.