How to attach an external USB drive read-only

I would like to hook up an external USB hard drive and would like to do so in such a manner as to ensure that absolutely nothing is modified on the drive at all. Is there some way I can specify this before I connect the drive or while I plug it in? (e.g. hold down the shift key or something like that.) I have found all sorts of posts from folks who wanted to know how to turn off read-only mode, but haven't had much luck finding out how to turn it on.
I noticed recently when I plugged in a USB stick that it sat and blinked for some time. I'm guessing that OS X was busy indexing the drive. I'm not sure if the index was written back to the USB stick, or cached somewhere locally. If it was written back to the USB stick, I would like to prevent this from happening when I plug in a real drive.
Thanks for any tips you can give me.
--wpd

If the drive is formatted NTFS, it will automatically mount read-only. If it is formatted HFS+ or FAT, I don't know of any way to external drives to mount read-only without installing OS X Server for $500.

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