A bad hard drive is a distinct possibility. Even though SMART is fallible, you can use Disk Utility to get a result. Better still, run a surface scan with TechTool Pro, TechTool Deluxe or Drive Genius, if you have one of them. If Applecare is still in force, by all means use it.
Jon
macOS 11.7.10, iMac Retina 5K 27-inch, late 2014, 3.5 GHz Intel Core i5, 1 TB fusion drive, 16 GB RAM, Epson SureColor P600, Photoshop CC, Lightroom CC, MS Office 365
| 
