It has to be FAT 32 with an MBR (Master Boot Record), and just one partition which must be a primary (not logical or extended) partition...
[edit 09/22/2020 00:00 EDT]
I wanted to add that I have found many USB flash drives that hang on for deal life to any previous partitioning/formatting they might have had--I have found this typical of USB drives previously formatted to be ISO image boot devices. To reset these you may/probably will need a hard core partitioning tool like EaseUS Partition Manager or Minitool's PartitionMagic.
Linux's command line utility fdisk and other Linux disk repair utilities are also adept at hardcore recovery of such devices. Here is a good tutorial on doing just that.
Here is another...
[/edit]
[edit 09/22/2020 00:00 EDT]
I wanted to add that I have found many USB flash drives that hang on for deal life to any previous partitioning/formatting they might have had--I have found this typical of USB drives previously formatted to be ISO image boot devices. To reset these you may/probably will need a hard core partitioning tool like EaseUS Partition Manager or Minitool's PartitionMagic.
Linux's command line utility fdisk and other Linux disk repair utilities are also adept at hardcore recovery of such devices. Here is a good tutorial on doing just that.
Here is another...
[/edit]
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