Missing Startup Disk
My computer had been running slow and lagging, instead of reinstalling from a backup I wanted to erase all the data and start new. I erased my Macintosh HD drive through recovery as other instructions have mentioned, but now it won't recognize the StartUp disk on my computer. When the computer turns on its just a flashing folder with a question mark. It recognizes my WD Passport when I connect it and allowed me to download MacOS onto it through the online recovery. I don't understand why it won't recognize my internal hard drive. Please Help!!
iMac 21.5″ 4K, macOS 10.14