Issues with magnetic card reader on HP-41CV

[CheshireChris]

I have an HP-41CV in very good condition, so I thought it would be interesting to get a magnetic card reader for it. I bought one sold as "untested" because the seller didn't have a calculator to test it with, but no reason to suppose that it doesn't work.

When I connect my card reader to the calculator, writing seems to work fine. I wrote a short program, went to PRGM mode, put the card in and the motor whirred and I got a message saying something like "RDY 00 of 01". When I come back into normal
Execution mode, though, and feed the card I've just written back through the reader, the first time I do so it says "NO ROOM", and then on subsequent times "CARD ERROR".

Is this the correct way to read in a program from a card? Anything I should be doing differently?

Any suggestions for ways to test the correct operation of the card reader?

All advice very welcome.