In plugging a new 64GB microSD in I noted the file format was exFAT. Looked good till I noticed the card was unmountable in recovery mode (CWM or TWRP). On the card is where I planned to store ROMS, Nandroid backups and other large files like >4GB MKV video files...and that's the main thing - support for files >4GB. So, that leaves FAT and FAT32 ("The maximum possible size for a file on a FAT32 volume is 4 GB minus 1 byte or 4,294,967,295 (232−1) bytes.") out. So then Linux' ext2/3/4 file systems are possible. I'm trying ext4. It's visible in recovery mode but not mountable in operating mode. An app like EzyMount can mount it on the phone but it's inaccessible off the phone (via USB, so far, and that's the main problem) without removing the card and plugging it into a laptop with an ext4-reading app installed (like Ext2 File System). So then, am I on the right track with my file system of choice? And who knows a method of transferring files over USB to an ext4-formatted external microSD card in the phone? It's reasonable to abandon the whole ext2/3/4 idea as well as storing ROMS on the external card (keep 'em on the phone memory which is FAT as I recall) - just go back to exFAT. Not as much fun though I should mention I'm a n00b 'Droid user with no Linux experience.