I’m building my first Zynthian these days (the 5.1 kit) and I need to get an SD card for it. My question is, is there any point getting a bigger SD card, say 64Gb or 128Gb, or is that just a waste of money?
I’m aiming for a 32Gb SanDisk (class 10) as I have read that this should be enough.
It’s not really needed for the system itself, but if you want to add any soundfonts or do any kind of audio recording type stuff, grabbing a large ish card wouldn’t be a bad thing. But also, yeah, probably a 32 gig card should do very nicely, unless you’re planning on doing any particularly large amount of recording type stuff 
It is advised to do audio recording on a separate USB anyway. But yeah, soundfonts can get quite large.
Any issues with larger capacities? I have read here and there that people get booting problems with 128 Gb and up… but they often turn out to be due to some other problem, or?
128GB and up could consume too much power from the Pi5, especially fast cards, resulting in unreliable operation. YMMV. Cards this large will work better if connected directly to the Pi5 and not using the extension.
A SanDisk 64GB Max Endurance or Extreme Pro will work fine anyway. I’d prefer the Max Endurance, because of endurance, because there are many writes happening during use of the Zynthian. The high speed features of the other do give a tiny bit of an advantage, but not enough to outweigh the benefit of greater reliability.
NVMe is the best in all aspects.
This should not be the case. Zynthian was designed to minimise writing to the uSD card. Do you have some evidence of this behaviour. We would want to investigate such behaviour.
Automatic Snapshots, samples saving, Midi recording, pattern saving, audio recording, …, updates.
It is always better to use external USB for that, but sometimes the internal memory is just there and not at home in the desk drawer, and not consuming a USB port.
I initially had that boot issue you described but when I switched to a 32Gb SanDisk extreme (class 10) it worked perfectly.
Sounds like a commercial 
Ok, thanks for the help.