Dennis is correct, you do not even need either ESP or bios_grub on a data only drive.
Its just I have no data only drives. Or even if planned now to be data only, I may want a system on drive later.
With new drive sizes, both ESP & bios_grub are tiny.
Installer will not create both, but if installing in UEFI mode it will use ESP for UEFI boot.
If installing in BIOS boot mode on a gpt partitioned drive, it will use bios_grub. If drive is pre-partitioned, it may not add partition or if install is to sdb or external drive it may look for ESP or bios_grub on first drive and then throw error that it cannot install grub.
I put both on for years as I was converting from BIOS to UEFI and thought I may move a drive from one system to another.
Some reasons to have a system on every drive.
Creating a Dedicated Knoppix Partition for large drives
http://www.troubleshooters.com/linux..._partition.htm
Except I have multiple Ubuntu installs and keep a current testing one on every drive, but smaller partition if just for emergency boot.
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