By coincidence (?), same was discussed at the Berkeley forum just days ago (same title). Don't do this to circumvent security, plus if the CPU/OS is not compatible, you run risk of corrupt work. Only run BOINC on computers you have rightfully been permitted to run none standard software.

Now how to do, with all these warnings is to get an older BOINC version that can be installed to one place. The newer versions spread themselves around and wont even allow to be put in 1 place. E.g. 5.10.45, see Start Here forum for client info, can do this.

And USB sticks have limited lifespan, so set the write to disk option to a longer period and activate checkpointing so you know when to stop BOINC.

Want to know more, look for Sneakernet set up.

It's a nice solution, if you can boot the device into Linux. Most are stuck to Windows.