TextEdit, can't save ".txt" files

Background Introduction:

I updated my system OS to "macOS Catalina" on July 10th, 2020.

I updated Tuxera to read my external hard drive.

I have granted "TextEdit" the full access to all my "Files and Folders" in "Security & Privacy".

Get info of the txt file is in full capability of Read and Write.

Problem:

Before today, I could use TextEdit to edit the "*.txt" files in my external hard drive.

Today, I copied several "*.html" files to my "External Hard Drive" and used TextEdit to make some modifications. After saving it, I used "Chrome" to open the HTML file to check. Again before it was everything alright, but right after I used TextEdit to modify the .html file again and saved, and reopen it in Chrome, Chrome shows that: the page is not found.

Thereafter, every txt file in my external hard drive can be opened with TextEdit, but couldn't be saved after editing. I have to copy the .txt file into my local drive and make the edit and save, then copy the file back to my external hard drive.

Plz help me solve this problem.

Thank you.