This may be more of a linux question, but I’m relatively Debian savvy and I can’t sort out what’s happening.
The output of df shows the sd card full. I’m not keeping anything on it other than the OSMC backup, which currently consumes 652MB, so I’m at a loss to understand what happened to the rest of the space.
Well there are at least too hidden directories which could contain many files.
So recommendation is install ncdu (sudo apt-get install ncdu) and run ncdu /media/osmc_sd that will give you all the info needed.
Its probably a hardware problem and the SD card has failed. Try another card, or try a full reformat of your current SD card (where bad blocks/sectors are marked as do not use) and then check the identified size.
parted sets up the partition tables, it doesn’t set up the file system (i.e. format) & potentially check the integrity of the blocks allocated to those partitions. You can do this with other commands such as ‘badblocks’ badblocks(8) - Linux manual page and ‘fschk’ fsck(8): check/repair file system - Linux man page. Its ‘simpler’ on Windows, where the (full) format command can be used to set up the blocks of a partition and mark them bad as necessary.
I would suggest you try another SD card and see if you get the same problem, to eliminate the possibility of a hardware problem.