Trouble migrating us

Hi - here’s what I’m having a problem with: I am trying to use migration assistant to add a user account.  The receiving computer is a 2014 iMac using HS. The older computer with the user I want to transfer is probably using ElCapitan. Not sure, as I have the internal drive from the older computer removed and being used as an external drive connected to the iMac.

When I open MA, it shows 3 users on the external drive: a square user icon, a black upper body icon (account?) and something called PostgreSQL. I “chatted” with apple advisor this morning. They did not clear up why there were 3 available when the PDF on transferring shows only the square user icon.

Along the way with MA, I got to the page which says select information to transfer. The top panel showed the name of the account and I set a password. Below it showed the square icon and the PostgreSQL icon and gave me a temp password and asked whether I want to make them admins. I did not make them admins and hit continue. Then I hit a road block. Page said “PostgreSQL already exists on this computer.” (I couldn’t see it in accounts and users) The choice of replacing was greyed out. So I need to rename. In this case, I renamed PostgreSQL-new and that was accepted. I could not come up with a suitable entry for the line for user account name.

How to I transfer successfully onto the iMac?

Thanks SO much for any advice.  John