The Files app can read and write to four storage formats on iPhone and iPad devices: MS-DOS (FAT), Mac OS Extended (HFS+), APFS, and ExFAT. These are the formats that are supported by macOS and iOS, and can be used with external storage devices such as USB flash drives, hard disks, and SD cards. The Files app cannot read or write to NTFS or ZFS formats, as they are not compatible with Apple devices. NTFS is a proprietary format used by Windows, and ZFS is a file system developed by Sun Microsystems. To use these formats, the user would need to reformat the external storage device to one of the supported formats, or use a third-party app that can access them. References: = Find files on your iPhone or iPad in the Files app - Apple Support, The Files App on Your iPhone: Everything You Need to Know - MUO, The Complete Guide to Using External Storage on iOS and iPadOS - Gizmodo, What is the (file) format of iOS/iPhone apps? - Stack Overflow, How Apple's Files app is getting better in iOS 16 & iPadOS 16