Unable to save updates to a text file.

I am running macOS 10.15.7 on a MacBook Pro (16 inch, 2019) that I just bought. My previous MacBook Pro was recently upgraded to Catalina just before I made the purchase. So most of the files I will be using were crated in Mojave. Don't know if that is germane to this discussion.

I have decided not to do a full restore on the new machine from backups of the old machine, but rather to add folders/applications from my old machine as needed. I am the owner and only user on this laptop.

I have a thumb drive, which contains a text file that I need to update often during this process.

I open that file, make updates and try to save (Cmd-S). 

I get the message:

“The document could not be saved. You don’t have permission.”

When it tries to autosave, I get a similar message: “You don’t have permission to write to the folder that the file is in.”

I select the file in a finder window and choose File > Get Info.

I expand Sharing & Permissions. It says “You can read and write.”, but there is no lock icon.

I select the thumb drive in finder, which I assume qualifies as “the folder that the file is in”, then Sharing & Permissions. It says “You have custom access.”, but again no lock icon.

I am unable to save this document. It is very important that I be able to update this document quite often as I go through the process of loading folders and applications to the new machine.

What should I do?