Questions and Answers
1) I assume the linux SSD will not come formatted as ext4 from the
store. So what format should this be from the store, and how do I then
make it ext4?
SSDs meant to be setup as internal drives are shipped empty. That means you will have to first initialize it with a system of partition tables called GPT or MBR. For Windows 10 more modern GPT is required for UEFI. I prefer to have GPT in all drives rather than mix GPT and MBR drives.
An empty new SSD can be initialized as GPT either from Windows 10 using the disk management utility, or after booting from the Ubuntu Live USB using the The Try Ubuntu without Installing option and then opening the app Gparted.
You can also create partition(s) and format them as
ext4 from Gparted, but that is not necessary.
If the SSD is initialized as GPT and left unformatted Ubuntu installer will format it during default installation.
If this is the only disk connected during installation the Ubuntu installer will give you an option to erase everything and format it for you during installation.
If you choose the "Something Else..." (expert mode) you can partition and format the disk during installation.
2) What format should be the shared HDD so both Ubuntu and Win10 can
read and write files? My largest files do exceed 4gb, but those could
be worked on in SSD, and stored on an external drive if need be.
NTFS is the native format for Windows. Ubuntu can read and write to
NTFS formatted partitions out of the box.
exFAT is another possibility as the Linux kernel 5.4 (the default kernel in Ubuntu 20.04) supports this format. However, since this is rather new to Ubuntu, there may be some issues. For example, 20.04 and exfat
3) What format should the swap partition be?
Modern versions of Ubuntu uses a swap file by default. So you don't need a swap partition. A swap partition is not created during default install process. Linux-Swap has its own format in case you want to create one beforehand or during expert mode installation.
4) Is there a preferable format for external drives so the files can
be accessed on all linux, windows and mac?
NTFS seems to work in all three OSes.
5) Any advantages/disadvantages to dual booting separate drives, as
opposed to split partition?
If you have one fast drive and one slow one (say NVMe and SATA SSDs) then it makes sense to put both OSes in the faster drive. Thanks oldfred's comment above.
To be clear, not all SSDs are equally fast. Particularly, SSDs that use NVMe protocol are faster than SSDs that use SATA protocol. If your computer has an
NVMe SSD, then that may be faster than the
2.5" SATA SSD.
If you have two NVMe SSDs, then put Windows in one and Ubuntu in the other.
If you have two SATA SSDs, then put Windows in one and Ubuntu in the other.
If you have only one NVMe SSD and the other SATA SSD, install both Windows and Ubuntu in the NVMe SSD using the install Ubuntu side-by-side Windows option.
If the choice is between SSD and HDD, install both Windows and Ubuntu in the SSD using the install Ubuntu side-by-side Windows option.
The only disadvantage of putting both Windows and Ubuntu in the same SSD is that some future Windows updates/upgrades may overwrite the Ubuntu bootloader in EFI System Partition (ESP). When that happens the computer will boot straight to Windows. You will need to run
boot-repair from an Ubuntu Live USB to fix it.
Putting the two OSes in two SSDs allows you to have two separate ESPs, one in each SSD. This way Windows has its own ESP and Ubuntu has its own.
6) Does the USB need a specific kind of formatting for the Ubuntu
Ubuntu reads most USB out of the box just fine. Just plug and play.
Two Related Questions
Dual Booting win 10 and Ubuntu 18.04 on two separate physical ssds
Ubuntu Studio on Seperate Hard Drive
Hope this helps