How is this external formatted?
How would you make a FAT drive read-only in OSX - its not possible, so I don't think you can do that with a thumb drive
It is possible (apparently) not going to name the vender who say that they can do this (PM if you want to know. I don't want to advertise) & I will be checking out the software this week.
** edit **
Looked at a demo of some software & it is possible (& does work, just tested it) to make external USB drives no matter what format they are read only on a MAC. Love to know what witchcraft they are using to do this.
Without knowing the price of the software, it's hard to say if it's worthwhile. If it was a few bucks, or even maybe as much as $20, I'd probably pay for it rather than me trying to puzzle out how it's done, and possible breaking things or destroying data in the process.