WD my Cloud 6TB - Red Light Flashing

I bought an 6TB from ebay, and it turned out that the seller had removed the hdd from inside the casing, put in a dead HDD and sold it off.
I tried to de-brick / revive it using a 4TB HDD i have, using the guides available here. But when I power on the Cloud, it flashes blue for few seconds, and then flashes red light.
Wondering what do i do next

Is your 4TB HDD a Western Digital drive?

How do you know the seller did that? Was the drive inside not a WD drive? If there’s evidence of it then you could simply open a case with eBay, They’ll help you out. I’ve had issues similar to this with flash drives saying they have 256gb capacity but it being a fake. eBay got my money back ASAP and I still got to keep the dirve.

I’ve also read while researching a potential HDD upgrade on my My Cloud that it’s not quite as easy as simply swapping a HDD in a My Cloud, the drive has to have the image on it prior, not sure if that’s what you meant by “using the guides available here”. If not this thread should help: Rebuild your My Cloud or upgrade the HDD on it

yes. its a WD Drive ( wd4000fyyz, enterprise drive).

I am assuming seller did it, no the drive inside was a dead 40GB seagate hdd. Yep i opened a case on wd and got the refund.
This is the guide i have used - https://community.wdc.com/t/guide-how-to-unbrick-your-2tb-3tb-4tb-my-cloud/93336
I have used it earlier successfully, with the same HDD and a diff my cloud PCB

That stinks. The guide looks good, so long as you are using a 4TB firmware for the 4TB drive, then I can’t see why you’d have a problem. Maybe buy another one off eBay and sell that one as “For Parts”? You’d be up a few bucks.