So I recently purchased a Seagate 4TB External Hard Drive (https://www.amazon.com/dp/B017KE8OG0/ref=psdc_595048_t1_B00HXAV0X6) and decided to put linux on it. My steps were (on Windows): 1. Converted to FAT32 and MBR (Maybe an issue?) 2. Created an Easy2Boot partition (I tried some others on Mac and Windows but it didn't seen to work). 3. Added a bunch of linux distros. So this worked fine on my Windows computer and ran like a charm. However, I would like it if I could run it on my Macintosh computer and (if possible) run it on my Windows without changing anything if I wanted to. When I plug this into my mac, I can look under the startup disks and the boot options (when you hold down option during boot) and nothing is found. How could I fix this?
Macs can only UEFI-boot AFAIK (you don't say what type you have). You need to make a .imgPTN file and add it to the Easy2Boot drive. As a first step - try converting a clonezilla.iso. Read more about UEFI-booting and watch the YTube videos.
Also, the maximum E2B will support is 2TB (2TB of your drive will be wasted because it does not support GPT partitions). I do not recommend a very large FAT32 partition. I recommend a 137GB NTFS partition for E2B. You can then add >4GB .imgPTN files and .iso files if you wish.