Epson Progression U12 - Manual (Page 210)

Write-protect
To protect the data on a diskette from being changed by placing
a write-protect tab over the notch on a 5.25-inch diskette or by
setting the write-protect switch on a 3.5-inch diskette. You can
also write-protect a file on a disk by using software to designate
the file read-only. When a diskette or file is write-protected,
you cannot erase, change, or record over its contents.
Glossary 23