Sony PCG-GRT100K - Manual (Page 240)

Why is the drive unable to write data to a floppy disk?
Make sure the floppy disk is properly inserted in the drive. See Using a Floppy Disk Drive for instructions on how to insert
the disk properly.
If the disk is inserted properly and you are still unable to write data to it, the disk may be full or write-protected. You can
either use a floppy disk that is not write-protected, or disable the disk's write-protect feature by moving the write-protect
tab into the "close" position.
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