This is the solution that worked for me to resolve the "Error: Resizing Existing FAT Partition" message

1. Open Command prompt with administrative privileges
2. DISKPART
3. LIST DISK (You need this to find the Disk # of your USB Drive. On most machines, it will be "Disk 1”)
4. SELECT DISK 1
5. CLEAN
6. CREATE PARTITION PRIMARY
7. SELECT PARTITION 1
8. ACTIVE
9. FORMAT FS=FAT32 QUICK
10. ASSIGN
11. EXIT
12. Unzip Recalbox.zip to the USB