If some utility of backup software created a small partition on your flash dire, there is no way to access the rest of the flash drive unallocated space. It is completely inaccessible. To reclaim the space, you need to delete the small portion using utility DISKPART.
Start CMD as Admin
select disk 1 (if that your desired flash drive)
detail disk ( make sure that this is the partition that you need to delete
clean (DiskPart succeeded in cleaning the disk.)
EXIT (Leaving DiskPart…)
This last step (CLEAN) will delete all partitions from that disk. From that point you would be able to manage your flash drive using regular diskmgmt.msc.