I have tried both sfc/scannow and chkdsk to see what problems I'm having on my admittedly failing HD. However, as soon as I click on the OK button to start them, they flash for a second on the screen, then disappear. Yet some things do run, such as REGEDIT and MSCONFIG so it seems to be selective. What could cause this?

For sfc and chkdsk you need to run them from a command prompt window. SFC requires an elevated command prompt, run as administrator.

https://www.howtogeek.com/194041/how-to-open-the-command-prompt-as-administrator-in-windows-8.1/

Yes I know all of the above and still they disappeared after a brief flash. I've sold the problem now by rebooting; it seems I need to reboot before everything. Time to backup and fast.

I've been using AOMEI but it's slow and I'm not sure if it makes a bootable copy (never had to find out). Is there anything better? At least there were no errors detected on the HD according to SFC so it might just be the HD that's wonky.

Snapshot is an excellent very small image program that plain ole works. Saved me many of times.
http://www.drivesnapshot.de/en/down.htm

HP Envy 750-520,Win 10 Home X64,Intel I7-7700, 8GB DDR4,PC4-19200,Radeon RX 460(2GB GDDR5 Graphics)256GB SSD/1TB Hard Drives