I have several partitions and I just want BOINC to use one of them when it runs. Is it possible and how do I manage that?

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All of the BOINC applications run within the subdirectory where BOINC is installed.

If you were setting up a new system, you could just select your install directory for BOINC to be on the desired drive. All science applications you attach to will run from that same drive.

If you have a system that is already installed, you can do the following:
Exit BOINC (File -> Exit, to completely end it)
Copy the entire BOINC folder and all subfolders to the desired drive.
Change your BOINC shortcut to point to the new drive and subdirectory.
Restart BOINC.

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Running Microsoft's "System Idle Process" will never help cure cancer, AIDS nor Alzheimer's. But running Rosetta@home just might!
https://boinc.bakerlab.org/rosetta/

It does just use one of them (unless your page/swapfile is on another or multiple drives)
It, as mentioned above/below, uses the one where you installed it, by default (for Windows) it is /Program Files/Boinc

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Team mauisun.org