Linux operations on Dockstar

I am recently interested in the lifespan of NAND memory used inside devices like Dockstar and in USB drives.
My question is: For regular debian linux installation on USB drive as we are doing, does debian only do reading from the USB drive or also writing to the drive (besides swap space access)? A related question is if I want to install a read-only system, can I install on an external USB drive as well? The 256MB internal memory maybe too small for some applications.

Debian on a Dockstar or similar device is very much like running Debian on an NSLU2 (aka "slug"). I've been running Debian on a slug for years, with the OS on a flash device.

You might have a look at this, it has some tips for optimizing Linux for running on a Flash device:

http://www.cyrius.com/debian/nslu2/linux-on-flash.html

I'm running ubuntu on SheevaPlug from the beginning (I got it within first 3 months of its avaliability in Europe) and It's installed on NAND, but I moved most used folders of the file system to a pendrive.

The lifespan of a pendrive is higher or equal to NAND memory we got inside dockstar, but one can not plug in a new NAND when the old one fails. That's all. A well managed flash memory should handle milions of erase/write actions before a single block gets damaged.
It's just better not to use up the internal memory, but I wouldn't worry about any pendrives. It's more likely that the pendrive would fail due to its own issues (some pendrives just die...) than out of being written too much. Though I wouldn't recommend creating a swap space at all if you are planning on using the device heavily and want it to be reliable for years.

And I've read somewhere that when you use up the read/write cycles the pendrive becomes readonly - this means you can still read a used-up block.

So don't worry too much.

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"And I've read somewhere that when you use up the read/write cycles the pendrive becomes readonly - this means you can still read a used-up block."

I wouldn't count on that (or the place you read it). You certainly do not want a system running on anything that's less than optimal, and not all flash devices behave similarly, nor do all systems that mount them.

There are extra blocks on almost all flash devices, which can be used to replace any bad ones. Flash devices use "wear leveling" to try and even out the usage, so when you think you are repeatedly writing to the same location you really are not, the wear leveling moves things around to even out the wear. That's an oversimplification, but I hope you get the idea.

The amount of "spare" space and the quality of the wear leveling algorithm is what differentiates the better flash devices from the lesser ones (speed is another).

So if you ever wondered why two apparently similar flash device have very different prices, that's the reason.

Unfortunately the *actual* specs on a particular flash device are guarded more carefully than most nuclear weapons access codes.