This is also mentioned in the recent thread: https://forums.mirc.com/ubbthreads.php?ubb=showflat&Number=174607

Sat also has a utility at http://www.xise.nl/mirc/devices.zip that shows which device names are blocked. mIRC has no way of knowing which device names will put you in a world of hurt if written to, so it plays it safe and avoids writing to them.

Perhaps a better solution is to change the error from

* /write: unable to open file

to

* /write: unable to open reserved filename