I need to first say I have no experience with that burn in software, so I can't give any advice specific to that. With that being said it could be a few things. Obviously the drive could be bad, but it appears to have zero errors indicated so far. If these are shucked drives, it is possible that the one drive is a different model. When brands make external hard drives they will use multiple models, it is possible that 7 are a higher class model and the slower one is a lower class or otherwise different. Again I'm not familiar with the software, maybe something weird is going on with it and it is hitting that drive less. You might have an old or damaged sata cable that isn't allowing it to achieve full 6 gb connection. Some motherboards can have wonky setups with their sata controllers, maybe something is going on with that sata port.

I'd allow the test to complete, if it has errors then you know it is likely bad. Do a speed test, if it is noticeably different then try switching the sata cable from one drive to another to see if the problem follows the cable / port. Make sure all drives are formatted the same, maybe something funky happened with the formatting or the test runs different with different formats. I'd also check the drives with a different software like crystaldiskinfo and see what it says. It can tell you the rpm of the drive, firmware version, if there are any smart errors and more.