What it needs
The Digitakt plays 16 bit mono WAV files at 48 kHz. Anything else is either converted on the way in or refused, and the conversion is not the one you would have chosen.
Mono is the constraint that surprises people. The sample slots are mono, so a stereo file does not become a stereo sample, it becomes a decision the machine makes on your behalf about which information to keep.
Format WAV Sample rate 48 kHz Bit depth 16 bit Channels Mono
Getting the files there
Samples live on the +Drive and get there over USB using Elektron Transfer. There is no card slot, so the desktop application is the route rather than one option among several.
Prepared folder on disk
Kick 01.wav 48 kHz 16 bit mono
Snare 03.wav 48 kHz 16 bit mono
-> Elektron Transfer, over USB
-> +Drive
Doing it in Sample Miner
Export Options has a Hardware target dropdown. Selecting Digitakt sets the format to WAV, the rate to 48 kHz, the bit depth to 16 and the channel count to mono, and clears the sampler format, because there is no instrument file for the Digitakt to read.
The downmix happens during export, so the files in your library are untouched. What lands in the export folder is what Transfer should send.
Mono is not a suggestion
A stereo file going into a mono slot loses a side or gets folded down, and which of those happens depends on what did the conversion rather than on anything you chose. Doing the downmix before the transfer is the only way to know what the machine ends up with.
This matters most on material where the two channels differ: a wide pad, anything with a stereo delay on it, a sample recorded with two mics. A kick drum recorded mono is unaffected either way.
The limits
The Digitakt is a drum machine with a sampler in it rather than a multisampling instrument. There is no key mapping to export, so a chromatic instrument does not travel as one object. It arrives as a set of individual samples that you assign to tracks yourself.
Sample memory is finite and 48 kHz 16 bit mono is not a small format. A library that fits comfortably on a laptop does not fit on the +Drive, so the selection is part of the work rather than something to postpone.
Sources
Specification confirmed against Elektron documentation.
Other devices: SP-404 MKII, Model:Samples, M8, Lofi-12 XT, EP-133 K.O. II.
What the files themselves contain is in the format reference, and the words each sampler uses are in the glossary.
To check a file you already have, drop it into the file inspector. It runs in the browser and uploads nothing.