Difference between "read-only" and "write-protected"

I'd say the difference is in emphasis.

You would use "write protected" if you were talking about preventing an accidental modification to contents. For example, modifying a floppy disk or a video/audio cassette so that it can't be accidentally over-written.

You would talk about "read-only" when it's a limitation. For example, you'd say "This word document is read-only" to indicate why you aren't able to make a modification you want to make to it.

Read Only

meaning you can't change what is on it

some of the disk which we have used comes with Read-Only mode, it means we can't change in that once it is written on it.

write-protected

Write-protected means you can only be read; you cannot write to them, edit them, append data to them, or delete them.

it is basically used for devices. like USB drive,floppy disk with device like USB you can not use word like

My USB drive is in read only mode.

but you can say with your files that is reside in your system.. that my file is in read only mode..

Well grammatically speaking, read-only excludes any action other than reading, whereas write-protected means you can do anything with it except writing, in your context I would not inmediatly know what other actions their could possibly be, but considering you are on english.stackexchange, yes, there is a clear difference in meaning.

Write-protected puts the emphasis on the user having made a conscious decision to prevent writes.

If you have a device that for some technical reason can only be read, and not written (e.g. a non-recordable CD) then it would be correct to call that read-only, but wrong to call it write-protected.

In common use (I have worked for many years in IT security) read-only is used much more commonly than write-protected.