M1 iMac not detecting USB keyboard A1243

That particular keyboard is over a decade old and obsolete. The issue could be it's downstream USB ports which the newer iMac has an issue with. At any rate, your M1 iMac came with a TouchID keyboard, why not use that?

Well, you have a spanking new TouchID keyboard made for your spanking new iMac that works both wired and wireless. So my recommendation is to use it exclusively. I wouldn’t count on a fix for wierd behaviors from an obsolete keyboard…all the best anyways!

There have been some unusual problems with some of the older Apple USB keyboards. Many people have found that if you use the extension cord that came with the keyboard the problems go away. I too use a 2010 vintage keyboard with my 2019 iMac and have no problems (but have always run with the extension attached as my desk set up requires it). I've never heard why the extension cord corrects the situation.

What extension cord are you referencing?

I much prefer the robustness and the slightly elevated angle on the Apple wired keyboard. I sold the TouchId extended keyboard for $185, reducing my final iMac cost. My wired keyboard works perfectly, but does not wake the M1 iMac from sleep. To do that I must press the on/off button.

Try triggering a wake by pressing the on/off button. Just a quick tap, not a press and hold.

I am referring to a 1 meter long extension cord that originally came with the Keyboard (upper left in the attached photo). I too am using the A1243 keyboard. I like it so much I bought a new spare a couple of years ago just as they were becoming scarce. You may still be able to find them. The extension is a little unusual in that it differs from a normal USB cord in that it has a male USB connector at one end and a female at the other end.

Disable Sleep, it’s unnecessary on later Macs, all pain, no gain. Shut for long absences i.e. > 2 hours. To disable, check “Prevent…” in Energy Preferences and uncheck everything else. You can leave the display off time as is, unrelated to Sleep.

Well, I went to my box of cables and there it was . . . the USB A extension cable that came with the keyboard. Now everything works fine. Thanks for the tip!

Great ! And you're welcome.

You might able to still find one that is new & unused. Try searching online at various vendors like OWC and on Ebay or Amazon. The mystery for me continues though... Why does the use of the extension help solve some issues ?

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. Reboot normally and test again.

NOTE: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

You can also try the following: Reset NVRAM or PRAM on your Mac - Apple Support

and How to reset the SMC of your Mac - Apple Support