Epson 3865X Plus - Manual (Page 84)

To write-protect a 5 diskette, cover the small,
rectangular notch (shown below) with an adhesive write-
protect tab. Write-protect tabs usually are included in a new
package of blank 5
diskettes.
kite-protect
notch
To remove the write protection, peel off the write-protect tab.
Note
Some program diskettes, such as your MS-DOS diskettes and
Using Your Computer 3-17