fixing a corrupted SD card in linux
Well, that one is solved!

The problem was not the SD cards, but the fact that rooting etc. the Desire caused it to USB-brick. Never heard that before.

As Spezi77 advises in the presentation of his CM11-for-Desire link, I switched to the 4ext recovery, which has a special USB-unbrick function. Some sources also name fastboot commands that should do the same, but they did not work for me.

Long story short, I'm running Spezi77's version of CM11 on my Desire now!