Portable Hard Drive (USB) Locked, Not Able to Instal Big Sur

First of all, it sounds as though you have other files on that external? That is not a good idea and that may be the problem. If you want to install an OS, you need a bootable drive or partition; in other words, it needs its own space. If you just want to "park" the installer itself for safekeeping, you can do that without needing a bootable drive - in that case, you'd need to move the installer to the Applications folder of another bootable source (your Mac) and launch it from there for an install.

Second, is the drive formatted a) for Mac and b) with a bootable GUID partition scheme? Otherwise, it's a no go as well.

Third, most external drives come formatted for Windows and include some totally unnecessary software or apps on it - did you make sure you erased all of that when you first got it and then did the formatting above?

What you can try now is to temporarily park whatever files you've got on it now somewhere else. Then erase the entire drive, format it, and partition it into 2 partitions in Disk Utility. Name one "Big Sur" and the other "backup". Then try again to install Big sur; if that works, go ahead and move back your backup files.

You can't install on the installer. You are supposed to make the bootable installer on one drive, boot from that to install the OS onto another drive.

If your Macintosh HD is bootable, just boot into that and run the installer from the Applications folder. You can then choose that drive you created the bootable installer on.

Hello Helper7,

We understand that you're experiencing an issue with using an external drive with your Mac, and we'd like to share a resource that can help. You'll find steps specific to macOS Big Sur here: Use external storage devices with Mac

You should be able to use any external storage with these steps without installing macOS on that particular drive.

Take care!