I believe it is correct that each instance of BOINC would use its own folder. However, that folder (or any other folder for that matter) could be given its own volume which could be spanned over multiple disks or drives. Please correct me if I am in error.
You could have multiple instances of BOINC installed on your system, each in its own unique area. However, I doubt if you could then run multiple instances of BOINC. That being said, if you ran several virtual machines, each virtual machine could have a separate instance of BOINC running, up to the point where you ran out of hardware resources.

Cheers

I don't want to create a new partition/volume or whatever. I just want Boinc to use the free space on my partitions. That free space changes constantly, depending on where i put downloaded files or delete files i'm done using or move stuff around. Plus, from what i've read if you use more then one disk for a volume, any disk failure means you lose the whole volume.

Trying to kill a fly with a sledge hammer. Uninstall BOINC, move the data directory to a drive location you know has piles of space [5GB is a pile for any regular machine on WCG]. Reinstall BOINC, pointing the data directory path selection to the location you copied the data to.

Default the path on Windows is something like C:\ProgramData\BOINC, the path printed in the Event log.

Make it

?:\BOINC

Never use root dir, never share a directory with any other programs as BOINC counts anything it finds in it's way.

Sometimes the projects aren't very good about cleaning up files from old work; you can safely do a project reset if you don't have any currently crunching tasks for it and the files will be cleared out. You could also go through and remove files manually if you prefer. The project will re-download any files it needs to do current work, that are not present.

Just don't do it while a task is running, that will likely result in an error.