What would happen if I delete the EFI partition and reformat as MBR partition table on an external drive not booted from?
Nothing. It would still 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 (232 512 byte sectors), or 2.20 TB.
The GUID Partition Table (aka GPT), on the other hand, is a more modern way of dividing a drive into partitions. It allows for drives larger than 2 TiB, and it allows for way more partitions (up to 128 specifically).
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 EUFI.
There are three catches here though:
- If you use ancient software (e.g. Windows XP, which came out before the GPT scheme was created), 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 partition a drive larger then 2.0 TiB with the MBR scheme, you will not be able to access anything past the 2 TiB mark. Your 1 TB drive, however, is small enough that this does not matter.