Howdy. Your DNS323 hit the
94% problem, when the first drive went in and did not complete, a bug in the firmware did not update the internal file tracking the state of the hard drive. So, my guess is that it is expecting the first 1.5TB, therefore is not mounting the second.
One thing you ought not do is keep switching firmware version, but pull all drives out, then reset the unit back to factory. Then try putting in the second drive, and this time, if it ask you to format it, proceed, but expect it to hang at 94%. You should leave it there for at least 2 hours (given it is 1.5TB, in order for the format to really complete). After which, recycle power, when you get back to the web configuration page, click "
Skip" when it ask you to format the drive. If DNS323 still refuse to detect your second driver, it is likely to be a defect, either the drive or the slot.
Once you hit and circumvent the 94% problem, every time you logon the web configuration, I suspect it will still ask you to reformat the drive. Each time just remember to skip. Until Dlink fix the bug thoroughly, for all intend and purposes, the underlying file system is intact for simple file operation, but some other services may choke - for example, DNS323 will not allow you to install APKG under this would-be condition.
p/s: See
http://forums.dlink.com/index.php?topic=11903.0 for my account of recovering from degraded RAID1, where some information explains your encounter.