Hmm looks like it's the Nas.
I plugged the faulty second hard disk into the third slot
And the third slot hard disk into the second slot
Second slot refused to light up and detect
Third slot with the "faulty" second wd hard disk lights up immediately.
Thanks all.
Black Fri coming soon haha.
Posted from PCWX using POCOPHONE F1
There's a possibility it's just the Bay 2 SATA adapter/connector that needs replacing. If so, u can call distributors like ACE Peripherals and ask them to quote how much it'd cost to replace... I've replaced once b4 for my DS413j cos of similar issue.