What OS are you running? Some Operating Systems (depending on updates) will not recognize ssd's. Could be the BIOS may need to be updated to accept ssd's. The motherboard may not support ssd's. Go to the manufacturer's website and see if there is a special driver you need to install to accept ssd's . Or maybe there is an update via the operating system, bios. It could be you just need to install the drivers for it. But first check with the manufacturer to see if your Asus will accept the ssd as per your particular model. And check the bios configuration to see if there is an option to enable or disable ssd's. I know an Asus I used to have had that option in the BIOS. But that was only after a BIOS update. And by default it is disabled.
Are you using a HDD caddy? It is very important to use one... It keeps the HDD locked in place at all times... I had an issue with that too... I was reluctant to get a caddy myself, but it turned out to be fix.
Also, I've noticed that using hard drives that are identical to the master drives usually don't work for some reason... But it may just be a strange experience with my Toshiba HDDs.
Check Device Manager, see if you see the hard drive labeled there...