Understanding Disk Utility: external drive returns I/O error but Disk Utility says it's OK Having recently purchased Backblaze, I'm currently backing up to the cloud my 4TB WD external drive (7 months old). The backup process can run without problems for a few hours (ranging between 4 and 10 or so), but will inevitably stall on I/O errors with the external drive. Running First Aid using Disk Utility returns that everything is fine. I've tried a different USB port and a different USB cable, all to no avail. 

Obviously the process of backing up the entire drive is accessing sectors that I might otherwise not have accessed for a long time, and which might be corrupt. But why does Disk Utility say the drive is OK? Does it only scan the headers etc.? Might the problem actually be with Backblaze? Is the best solution here to exchange the external HD under warranty?
FWIW: As suggested by @Tetsujin, running a more 'aggressive' tool over the drive (in my case TechTool Pro) uncovered a number of bad blocks.