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I can't see anything you did wrong.
"Going read-only" usually means that the file system has been corrupted. That shouldn't happen unless the device force-reboots in the middle of a transfer, the format didn't "take", or the SD card is failing. The last one is the most common. However, to have it happen to 2 SD cards makes me wonder if it isn't the SD cards but is instead the device itself.
If you have a card reader or something that will allow you to insert the SD card into a desktop/laptop then do a disk check on the card. You could also format the card using the desktop.
Also be sure to reboot the device before inserting the SD card, just in case, in order to help android avoid confusion.
In CC, if you change SD cards then it is common to need to re-select the CC folder using CC's Settings / Formats and folders. This is required because Android security is card specific, and CC's access permission for one of them won't work on the other, nor will it work after reformatting.
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