I'm gonna add another hard drive to my system to dual boot another OS but in doing so, I want to disconnect my existing hard drive which is my main hard drive for doing everything. If I temporary disconnect the cable to the main hard drive and then plug it back in later when I want to use it, will it make it lose all its content and make it corrupt?
Because when installing the other OS on the other hard drive I don't wanna have both hard drives in at the same time just incase I make a mistake.
So is it safe for me to disconnect my main hard drive for now and just plug it back in later and use it like how it always was?
Yes, it's quite safe to disconnect a hard drive. It won't affect it's contents.
Ordinarily, when installing Windows, it's always the best policy to disconnect all the other drives.
However, if your drive contains Windows and you want to be able to have a dual boot setup with another OS on a separate drive, you should not disconnect it. If Windows Setup can't detect an existing OS (because the drive has been disconnected) it can't set up your system for dual booting. You won't have a dual-boot menu, and without that you have no way of choosing which OS to run whenever you turn on your PC.