USB drives could use such a feature.

write-lock: prevent it from beint written (and acquiring viruses)

read-lock: prevent it from being read (and passing viruses), only showing the free space and being able to write only in the free space

I have a USB stick with a write-protect switch. Unfortunately, the switch has broken. Now the drive will contain 128MB of obsolete early-00s data until the bits diffuse away.

SD cards have a write-lock. It always amazes me that it's purely physical and only a suggestion to the software.

Gotta love how the write-lock switch is positioned just perfectly so that inserting it knocks the switch into read-only position.

Doing that rather than switching into write-allowed position is a good example of fail-safe design. But yeah, I've had that happen more than once.

On some SD cards in some SD readers, the write-lock switch is too small to trigger the mechanical switch in the reader, making the card appear as read-only until tape is applied over the switch. This problem affects my BeagleBoard's OS SD card.

While not built into drives, an add-on hardware solution exists, Wiebetech's USB Writeblocker:

http://www.wiebetech.com/products/USB-WriteBlocker.php

I have several USB drives with at least the former feature.

I only ever had drives with write locks. I must have gotten into USB drives late because I've never heard of read lock.

We did this on VCR tapes too. Little electrical tap over the broken tab, and bam -- you've got re-writable media!