Fresh install of Pi, old data from HDD.

I have a PI 4 currently running with some stuff on it, to it I have also connected HDD with data in it. I want to do a fresh install on Pi, however I don't want to lose the data on HDD and have no place to temporarily store them. Apparently, unplugging the disk, doing the install and plugging it back in may be enough for Pi to load the disk, however I'm not sure about this. Another thing I worry about are permissions if the above method works. The Pi OS and everything it needs is on SD card.

The rights is preserved on the HDD and if you need you can change them.