You can upgrade back to 6.7.1. That's not the problem.
Plug in the T1 to the USB and try running a chkdsk on it and see what happens.
The problem could be that the T1 is failing. It is 11 years old. If it is the case that it's failing, I suggest you go for a Kobo Libra 2 as a replacement. I was using a T1 when I switched to Kobo.
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