iMac magic trackpad and keyboard non working in bootable usb

[paolocimenti]

Hi! I was trying to bring my late 2012 iMac back on high sierra from catalina to use it as a monitor with target display mode. I created the bootable high sierra, it shows up in the boot options but when I click it and wait for it to be ready, it goes on a screen asking me to connect keyboard and mouse. The magic mouse and keyboard usually work flawlessly, connecting automatically, and even other cabled peripherals don't work. Please help!

 

[DeltaMac]

Try a deep reset - NVRAM reset: shut off your iMac, then power button to boot, while holding Command-Option-P-R
Keep holding those 4 keys. You will hear a boot chime. Keep holding the same 4 keys until you hear the boot chime 4 more times.
Release the keys, except continue to hold the Option key to get the boot screen. Choose your bootable installer.
I would suggest your best option would be a wired USB keyboard and mouse. If you still get the "can't find a valid input device, give me a bluetooth wireless mouse to try" (which is what that popup is asking), and the wired USB mouse and keyboard doesn't help, then you likely have some other issue happening, like a bootable installer that is somehow incorrectly made.
(hint: connect a wired USB keyboard and mouse BEFORE you try to boot, to make sure that hardware is part of the boot process.)

 

[paolocimenti]

Try a deep reset - NVRAM reset: shut off your iMac, then power button to boot, while holding Command-Option-P-R
Keep holding those 4 keys. You will hear a boot chime. Keep holding the same 4 keys until you hear the boot chime 4 more times.
Release the keys, except continue to hold the Option key to get the boot screen. Choose your bootable installer.
I would suggest your best option would be a wired USB keyboard and mouse. If you still get the "can't find a valid input device, give me a bluetooth wireless mouse to try" (which is what that popup is asking), and the wired USB mouse and keyboard doesn't help, then you likely have some other issue happening, like a bootable installer that is somehow incorrectly made.
(hint: connect a wired USB keyboard and mouse BEFORE you try to boot, to make sure that hardware is part of the boot process.)

thanks, it worked fine!

 

[paolocimenti]

Try a deep reset - NVRAM reset: shut off your iMac, then power button to boot, while holding Command-Option-P-R
Keep holding those 4 keys. You will hear a boot chime. Keep holding the same 4 keys until you hear the boot chime 4 more times.
Release the keys, except continue to hold the Option key to get the boot screen. Choose your bootable installer.
I would suggest your best option would be a wired USB keyboard and mouse. If you still get the "can't find a valid input device, give me a bluetooth wireless mouse to try" (which is what that popup is asking), and the wired USB mouse and keyboard doesn't help, then you likely have some other issue happening, like a bootable installer that is somehow incorrectly made.
(hint: connect a wired USB keyboard and mouse BEFORE you try to boot, to make sure that hardware is part of the boot process.)

another thing, I managed to reinstall high sierra but I cannot activate target display mode. I am trying to stream from a pc with a displayport to minisisplayport cable to the late 2012 iMac but it doesn't work. What should I do?
Edit: I think my cable is mini dp to dp, but I can't find a DP/HDMI to thunderbolt cable, could you link me one?(all the ones on amazon are only from mini dp to hdmi/DP)

 

[DeltaMac]

From this thread - https://macosx.com/threads/use-imac-as-a-windows-monitor.327241/
1. Your 2012 iMac would need a thunderbolt 1 connection to use for video in.
2. The source computer also needs a thunderbolt port, which leads to 3.
3. The 2012 iMac is too new for a DP connection, has to be a Thunderbolt cable. Even though the thunderbolt connector is also used for displayport video, Target Display mode on your 2012 Mac requires an thunderbolt cable
4. Finally, the source computer needs to be a Mac, too. Sorry, won't work from a Windows PC (which, to support the Target Display mode, would have to provide a native Thunderbolt connection, but again, PC's are not known to work at all with a Mac in Target Display mode, except for a few Macs (not all of them) from 2009 & 2010, before the iMacs offered thunderbolt, and the Target Display hardware supported the mini-displayport connection (no thunderbolt yet for those iMacs. Oh, and the only Target Display models at that time were 27-inch.
final note: to answer your question about HDMI. That won't be helpful, as the connection has to be Thunderbolt. There are no options