Just a quick heads up, the devs behind Macrium Reflect made a [IMHO] silly choice with their software when it comes to creating the rescue media you can use to boot the system for repair or to restore a backup. Again, just my opinion, I think the devs were going for something else that didn't quote work out, so they came up with a "Baffle them with Bull****" explanation.
The odd behavior is pretty straightforward... if you format the USB stick FAT32 first, creating the rescue media works normally. If you forget to check, want the USB stick to boot with UEFI BIOS, & the USB stick is NTFS, Macrium Reflect will create a 1GB FAT32 partition on the USB stick containing their software, while the rest of the storage is unallocated.
Practically speaking, there's just not much sense to that. If the entire USB stick was formatted FAT32, then you have the option of sticking something on there in the future, for whatever reason. You can't enlarge that 1GB partition, so to use that space you'd have to create another one -- something Microsoft says to do before you add any data to the stick, though that may or may not matter. I don't like them because they tend to be flaky in my experience, but USB sticks with multiple partitions are possible -- Microsoft has directions on how to do that for a WinPE USB stick, where logically you might use an NTFS partition to store anything from data to setup files to apps. With a USB stick to restore backups I haven't a clue what you might want to store on a 2nd partition. Given their slow write rate & questionable reliability, I'm pretty sure no one would want to store their backup archives there [that's when/where you'd plug in the external hard disk with your backups].