Found a great pen to mark HP calculator cards

[Xorand]

After recently purchasing a working card reader for my 41CX along with some new and used cards, I was debating what to use to mark my cards. I wanted something non-permanent, so I ended up using a mechanical pencil. Graphite, however, ends up smearing over time and to be honest doesn't erase too easily off of HP's magnetic cards.

On a whim, I tried one of my Pilot Frixion pens. I had purchased a few of these pens to go along with some re-usable notebooks I picked up a year or two ago. The ink dries within a minute or so on the HP cards and doesn't smear. However, the ink wipes off clean when wetted with some water (i just used a wet finger for a test).

I've got a few cards marked with the Frixion pen and will see how they hold up long term, as well as to see if they can still be wiped clean with water after a longer period of time.