Android: How do I copy files from SDCard_0 to SDCard_1?

Factual Questions
1

The impossibility of doing it is recent or I wouldn’t have 40 gigglebytes* of files on SDCard_1, but using several file manager apps I get errors about how since Android 5 add-on drives are read only and to work around it I’m to follow a procedure that doesn’t work.

  1. why would Android make the backup drive read only?
  2. I tried finding help online but all i could find were programming phones.
    • Idiot coworker kept calling gigabytes gigglebytes when we were supposed to sound authoritative.
2

Are you trying to move an app from internal to external storage? That was deprecated, but external storage isn’t read only.

Use SD cards in your Android device

3

On further research, read only status might mean the file system is corrupted or the sd card is physically failing.

Google “sd card read only”.

4

Just swapped out my 128gB for a 256gB SD card on this old Android 7 tablet.
It is decidedly NOT read only.
I have had SD cards go bad. You may be experiencing this
Try https://forums.androidcentral.com/