pressing the Option button (after the chime, before apple logo comes up) will let u pick where to boot (dvd, another partition, another hard drive etc) just like in the photo

pressing C means picking to boot from the dvd drive only

anyway yes WD blue got warranty, i've successfully RMA'd before.

since u dont have ur original RAM, try install back ur stock HDD. if can boot, then the WD blue may be faulty. u can also take out one RAM, n try boot. if can boot, meaning the RAM u take out is faulty

KTA is good but it's a third party RAM. kingston tests the RAM on macs, but apple never configured KTA as a mac's stock RAM

samsung RAM is normal. samsung is a huge memory supplier