Can I mount external SD card read-only? I have a SamsungGalaxy S3 (32GB Verizon version, not rooted) with a 64GB external SD card.  

I use the external card only for music, which I load from my computer. Several times now I have lost files from the external card. Is there any way I can force the S3 to mount the external card as "read-only"?  

Ideally, a widget that would allow me to toggle the mount from RO to RW and back would be perfect, but any way to mount it as RO will satisfy my need to protect the data on the card.
I disagree with Liam's answer.

Its possible with root, you can just use the "root explorer" app on play store and navigate to /storage/extSdCard/ and on the top, click the "Mount R/O" button to mount the drive as read only.

Surely if you know the linux codes, you should be able to make it into a widget for it :p (one line or so)

(I think there's an app that makes widgets from busybox codes) - that could be your answer