Hi Steve,
That is definitely an odd issue. The HDD SMART short test may not be able to complete properly if you have other programs accessing the hard drive at the same time, but it doesn't explain why the test hangs when ran from the 'Diagnostics and Tools' tab but completes successfully through the Script Editor. Are you running the diagnostics within Windows or using the Bootable diagnostics? Which version of Service Center are you currently running? Please let me know and we can work toward finding a reason for this behavior.
Thanks,
Channing