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Create a partition on your USB key using gdisk, with format type 0700

linux-vk9y:~ # gdisk -l /dev/sdb                                                                                                                                                             
GPT fdisk (gdisk) version 1.0.1

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with protective MBR; using GPT.
Disk /dev/sdb: 15466496 sectors, 7.4 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): F9AE86DD-375B-4496-9DBD-66F405323159
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 15466462
Partitions will be aligned on 2048-sector boundaries
Total free space is 2014 sectors (1007.0 KiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048        15466462   7.4 GiB     0700  Microsoft basic data

Now you can use the dosfstools to format

linux-vk9y:~ # mkdosfs -F 32 -I /dev/sdb1
linux-vk9y:~ # mount /dev/sdb1 /mnt