If this fifth drive would serve as a permanent addition, you can absolutely move them over and set up your system to automatically reference and refer to them on said drive instead of your system drive with ease.
Create “Documents” and “Downloads” folders on the fifth drive. Move your files to the appropriate, new folder. Then wipe them from your system drive.
Go to Finder and in your sidebar, right click on them one at a time and select “Remove from sidebar”. Open up the 5th drive, drag ‘n drop the new folders onto the sidebar and they will stick. They will now have generic folder icons, sadly, but it’s a small price to pay.
The real tedious work is going through your programs, like Safari, hunting through their settings, and redirecting download locations to the 5th drive. You, as well, have to remember now to be saving to these newer locations.
All in all, this is completely plausible with just a lil’ extra effort. It’s for your very space concerns that on my system, I always have a small SSD system drive, a 500GB-1TB “document drive” for all of those system folders, and a row of terabytes for projects and the like. My system drive only has the OS, my applications, and related libraries on it. I’ve had run into no problems keeping my custom document folders elsewhere.
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W.D. Conine
Digital Imaging Technician
Freelance Editor
Orlando, FL