I recently installed Ubuntu 19.10 on my desktop alongside an existing Windows 10 install; on a separate blank ssd. Here are some things the online guides dont tell you about dual booting.
UEFI
Does your system use UEFI? Yeah, I didn't know either. Cause I didn't need to. But to install Linux you will have to. Literally the first boot menu off the bootable Ubuntu usb will ask you to select from UEFI or non-UEFI options to install Ubuntu. Since most systems sold in last few years do use UEFI, you will need to select the correct option here.
Ok, so you told Ubuntu you have UEFI. Now you don't have to deal with that again right? Wrong. Ubuntu acts brain dead from here. When installing Ubuntu on a new ssd, you will need to manually create a separate EFI partition. The partition will need to be the first one on the disk and be 512MB in size. This is something none of the online guides talk about and something the Ubuntu installer should do or prompt you to do automatically, but doesnt.
Ubuntu does give you an option to 'install alongside windows' but that assumes that you want to install on same disk as Windows (in separate partition) and doesnt even consider that you might want to install it on a separate blank disk.
Time Syncing
When you boot back from your new Ubuntu install to Windows, you will notice its showing incorrect time. That's cause Windows reads the hardware clock as local time, while Ubuntu as UTC. The easiest fix for this is to tell ubuntu to also read clock as local time using this command:
timedatectl set-local-rtc 1
This is another thing Ubuntu could have atleast warned you about by just checking that Windows is installed on one of the disks, but it doesnt.
Fractional Scaling
If you have a 4k monitor and want to scale the display, you will notice that Ubuntu only offers 100% and 200% scaling and nothing in between. You will need to enable the 'experimental' fractional scaling feature to get other options like 150%, 175% etc.
However, after you actually use the fractional scaling and reboot into your system, you will notice that all the display properties have been reset. That is due to this bug I discovered. So for now, you will need to reconfigure your display settings each time you reboot.
Conclusion
Ubuntu on desktop remains a mess. Installation requires you to understand intricate details of your system like UEFI partitions, which is completely unnecessary and should be taken care of by the installer (like the Windows installer does).
The fact that 'Fractional Scaling' is an experimental feature, which is extremely buggy, and we are almost in 2020 just goes to show how much Ubuntu cares about the desktop experience.