Can I Merge Two Hard Drives with Different Operating Systems?

If it’s all too complicated, why not just strap the external drive to your laptop with some Velcro? Just a joke, but on a serious note, managing two systems can be tricky! Just ensure your setup allows for easy switching between the drives.

It sounds like you're looking to dual boot. Instead of merging the partition tables, you might want to create new partitions on the main drive and copy your files from the external drive over. Using something like Clonezilla can help you clone those partitions. Just make sure each OS has an EFI entry so you can choose which one to boot into! If you're running into booting issues, consider switching to systemd-boot instead of Grub; it tends to be simpler for multi-boot setups.

You could access the other drive from Linux without any issue, but Windows can be a bit more complex with Linux partitions. If you're set up right, you'll be able to pick which OS to boot using the EFI entries. If your external drive isn’t showing up when you try to boot, you might need to adjust some settings in your BIOS.

It's not advisable to have two partition tables on one drive. You should create a single partition table that can contain everything you need, and then transfer the contents of each filesystem appropriately. A little tweaking with your bootloader may be necessary as well. Since you mentioned you're using macOS, just ensure that your setup will handle the partitioning correctly.