What would happen if I delete the EFI partition and reformat as MBR partition table on an external (non boot) drive
Nothing. It would just work.
But you do seem to mix EFI and GPT, so let me give a longer answer:
MBR is a very old way of partitioning drives on a PC. It only allowed for four primary partitions, or, with a workaround, for 3 primary and several more extended partitions. It also tops out at 2.0 TiB (Which is the actual size in powers of two), often written as 2.20 TB in the powers of ten).
The GUID Partition Table (aka GPT) is a more modern way of dividing a drive into partitions. It allows for larger drives (> 2.0 TiB) and it allows for way more partitions.
There is nothing which stops you from selecting either method on a small enough drive. Both methods are valid. Any modern computer should be able to handle both of them, either via the BIOS or via EUFI.
There are three catches here though:
- If you use ancient software (e.g. windows XP which came out in 2002) then it will not understand GPT.
- If you use a old BIOS then it may not understand GPT. (A modern BIOS should understand it, though there are quite a few which fail).
- If you take a drive larger then 2.0 Terabyte then you can not access all of it via the MBR scheme. Your 1.5 TiB drive is small enough that this does not matter.