I can no longer transfer samples … I don’t think it’s a memory problem, so I take this opportunity to ask you how can I check the available space, and delete any samples?
Read section 15.2 (named “SAMPLES”) in the Digitakt manual. There you can see how you can delete samples from the +Drive. I don’t own the Digitakt, so I don’t know how to check the available space, but if you remove something and then try to add a smaller sample than the one you removed, that should answer your question.
Use https://electric.kitchen/crunch/elk-herd/ to view/edit/rearrange what’s actually going on inside your Digitakt.
You can check how much space is on the plus drive and the current project by going into the system menu. Accessed by pressing the button with the cog/gear icon.