@Plumadona,
You're not hopeless

there is a bug when using the combination of PRST1 & SD card & Calibre. There is a workaround which is discussed in the
main Calibre/PRST1 thread posts #302/303. The explanation of why the problem occurs is in post #317 of the same thread.
Once you have performed the workaround and got it all smoothed out, I believe it works fine for subsequent sends. I can't confirm because I don't use an SD card. By the way, these problems don't occur if you use Calibre to send to internal memory rather than the SD card.