El Capitan USB error Failed to copy kernelcache, “prelinkedkernel” couldn’t be copied to “.IABootFiles”.
yes yes, El Cap 10.11 is oooold (circa 2009 and earlier machines); and, yes yes, I'm answering my own question (but the correct question with a crappy "did you make your usb like this link" has been archived)
a Mac Pro from back then is still a useful machine, and I like sharing information (especially when I have to waste a half of a day figuring out why--hopefully this info will still help someone)
my old work around was to use an old El Capitan 11.15 installer to make the usb, but that trick stopped working (noting I grabbed the newest 2019 cert version of El Capitan from http://updates-http.cdn-apple.com/2019/cert/061-41424-20191024-218af9ec-cf50-4516-9011-228c78eda3d2/InstallMacOSX.dmg)
sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/untitled --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction
looks like all is going well, but then when it's done with the 6.2GB, it tosses the error:
Erasing Disk: 0%... 10%... 20%... 30%...100%...
Copying installer files to disk...
Copy complete.
Making disk bootable...
Copying boot files...
Failed to copy kernelcache, “prelinkedkernel” couldn’t be copied to “.IABootFiles”.
Done.
***SOLUTION***
make sure you're using the whole usb as one parition (and if you are, just try using a different brand usb); e.g. setup the usb like this (assuming it's /dev/disk4):
diskutil partitiondisk /dev/disk4 gpt jhfs+ elcapitan 100%
ok ok, I still wanted a triple installer for MacOS, so I forwent the first partition being windows (as windows can only see the first partition of a removable device); so, if you don't want to use the whole usb, you must at least make the El Capitan installer the first partition (don't ask me why, this is just how it is now for 10.11, but 10.13 and 11.0 don't need to be the first partition); e.g. setup your usb like this:
diskutil partitiondisk /dev/disk4 gpt jhfs+ elcapitan 6.6g jhfs+ highsierra 5.7g jhfs+ bigsur r