Problem trying to install macOS High Sierra on external drive.

(Apologies for the duplicate post- got out of synch and then couldn't delete the extra).

I went the terminal route as you suggested- (I was anxious about it before but persevered) and all went well, until I launched the installer, now installed on the external and got the same message.

RE the recovery server issue- I don't think that's one of mine.(?) My issue (of late) is with my iMac, (running Catalina OS - which I'm on now), failing to respond to any key commands on startup-- it just keeps brining up the login pane. I tried shutting down holding the power button>commands on start, unplugging the power/waiting etc...have run disc repair numerous times, tried nearly every command combo, except maybe diagnostic mode, and none work. The previous round of problems I had with the failing external SATA drive (using a usb adapter) on which I have been running High Sierra, before these other travails with the OS install, I was eventually able to get the disc option and recovery modes to activate. It took multiple tries sometimes and interim running of the disc repair (which was how I revived the SATA which has now finally evidently really crossed the rainbow bridge for keeps). I'm trying not to get too scrambled in my objectives, and prioritizing the High Sierra thing because I really need to use an older app on a project. At this point, I figured I may have to re-install Catalina on this iMac next to resolve the startup command thing. But I was anxious about burning any bridges while I'm still working on the other.

All wired except mouse. I googled the error and found an old thread on hear in which HWTech responded mentioning that there was a known bug and suggested to that person they download an older version then update to High Sierra. I attempted this last night- and was successful downloading the earlier version installer (I think I tried Sierra) and got to the point of install/opening the package, but hit yet another snarl; when opening the package skips over the select-destination and jumps to install (on my computer) not giving me the choice of other drives. I tried a few times, same result.

Deleted the installer(s) and re-downloaded (it was over 5g). Don't seem to have any other options.

Still getting this.

Thanks for this, and to all. Will make some more attempts. I'm on the verge of just buying another drive with it pre-installed, but I need this operating yesterday.