I had an ATI Radeon HD 4850 which ran Milkyway fine.

Then I stuck in an ATI Radeon HD 6790 in addition to this card (which I only have for a short while to test, as it is to go in a customer's computer), and Milkyway ran on both GPUs (after I told BOINC to use all GPUs in the cc_config.xml file), but the tasks running on the 6790 caused a computation error immediately on starting. However the tasks running on the 4850 continued to complete successfully.

I reinstalled the latest Catalyst drivers with no effect.

Then I removed the 4850, and Milkyway runs fine on the 6790 by itself.

Any idea what on earth is going on?

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