Cannot upload new files to iPhone

[UnitedMarsupials]

Though I was able to copy the entire contents of iTunes_Control/Ringtones/ from one iPhone-7 to my FreeBSD computer, loading the same files to another iPhone-7 fails:

# cp -p *.m4r /media/iTunes_Control/Ringtones/
cp: /media/iTunes_Control/Ringtones/EEFO.m4r: Input/output error
cp: utimensat: /media/iTunes_Control/Ringtones/EEFO.m4r: Input/output error
cp: chown: /media/iTunes_Control/Ringtones/EEFO.m4r: Function not implemented
cp: chmod: /media/iTunes_Control/Ringtones/EEFO.m4r: Function not implemented
cp: chflags: /media/iTunes_Control/Ringtones/EEFO.m4r: Input/output error
cp: /media/iTunes_Control/Ringtones/EHQH.m4r: Input/output error
cp: /media/iTunes_Control/Ringtones/EXYJ.m4r: Input/output error
cp: /media/iTunes_Control/Ringtones/GZHL.m4r: Input/output error
cp: /media/iTunes_Control/Ringtones/HPOV.m4r: Input/output error
cp: /media/iTunes_Control/Ringtones/HSBT.m4r: Input/output error
cp: /media/iTunes_Control/Ringtones/HZZJ.m4r: Input/output error
cp: /media/iTunes_Control/Ringtones/IGDD.m4r: Input/output error
cp: /media/iTunes_Control/Ringtones/JXCV.m4r: Input/output error

After those messages, any attempts to even read the mounted tree fail (with the same error), and it needs to be remounted again.

Upon remount, I'll see the first file with exactly 65536 bytes -- but no others.

Both iPhones run iOS-14, the ifuse is the latest master.