If you used to using Linux PC / Laptop, then best system file is ext4. Since Z1 didn't support NTFS regarding lack of kernel features (hopefully doomlord will add this on next build), then ext4 is the best choice.
But if you using Windows, FAT32 is your choice. But you can't copy a single file which larger than 4 GB (this is the lack of fat32), beside that fat32 can only formating a disk which not bigger than 32gb. In this case, you can create two or more partitions for your 64gb sdcard.
If there's any questions, let me know. Hope this help.
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