Unable to install Catalina on external SSD

[Ubele]

I have a 2015 256 GB MBP with the latest version of Catalina installed. I want to install Catalina on a 1 TB USB 3.0 external SSD, something I did in the past with Mojave. I have a USB flash-drive installer for Catalina. I boot from the flash installer, and it takes me to macOS Recovery. I sign in with my account, choose Install macOS (I already formatted the SSD as APFS), select the SSD as my destination drive, and begin the installation. It says Installing, and the progress bar goes on for several minutes. Then it automatically reboots. Intead of continuing with the installation, though, it reboots to the login screen of my MBP's 256 GB system drive. When I log in, the SSD doesn't show up as being connected. I physically disconnect and reconnect it, and it shows up. I check it with Disk Utility, and it shows 8 MB have been used. I erase it, and it shows that 897 KB have been used, so something obviously was installed. I've tried this process three times, with the same results. One time, I held down the Option key during the automatic-reboot stage, but it still rebooted to my 256 GB system drive's login screen. Any ideas?

 

[benshive]

Have you tried opening up the Catalina installer in macOS and installing it on the external drive through there?

 

[Ubele]

Have you tried opening up the Catalina installer in macOS and installing it on the external drive through there?

No, I haven't, but I will, and I'll report back. Thanks!

 

[Ubele]

That worked! There were a couple of unintuitive steps required, which I'll describe for anyone else who might want to do this: This time, after the automatic reboot, it went to the expected black reboot screen with the Apple logo, where it went through the second phase of the installation. Then it automatically rebooted again, where it usually goes through the final installation phase. This time, however, it went to my 256 GB system drive login screen. I logged in, and the SSD appeared not to be connected. I manually disconnected and reconnected it, and it appeared. I checked the SSD with Disc Utility, and the two expected partitions were there, so I went to Startup Disc in System Settings, and the SSD was listed there. I selected it as the default startup drive and restarted my computer. This time, the installation finished, and all is well.

Well, almost all. Now I can't figure out how to download the contents of my Documents folder to the SSD system drive. Documents is under iCloud Drive, but it has a dotted-line cloud icon instead of a solid-line cloud icon with a download arrow. I'm logged in with my usual Apple ID, so it should be synced. Any ideas?
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Never mind -- the option to download my Documents folder just appeared.