It's showing 0xaa and 0x55 as complete for all drives. It just completed 0xff and rolled back into 0xff again, so it's running multiple, sequential iterations of the same test before rolling over to the next variant. How many iterations, I'm not sure. So it's running the 3rd variant of four (0xaa, 0x55, 0xff, 0x00). Once that's done, we'll see if it rolls into the 0xff again or into the last variant 0x00. Yeah it's taking a long time. The good news is no errors - so far. Fingers crossed.
Is there anything more to analyzing badblocks data than reading the error count?
Note to self - be glad that I'll likely never need a bank of 18TB disks.