In general, microSD-Cards with UHS speed class I (U1) only work. However, during testing we have found that some microSD cards are not compatible. If you buy a microSD card via us, you are certain that it will work. We strongly recommend to (re)format your SD-card as FAT32/FAT.
UPDATE!: SanDisk ‘Pro’ and ‘Ultra’ cards do not work with Howler.