first aid on an external usb Hard drive isn't possible?

I have an external usb Harddrive which I use on a regular (=daily) basis without any issues whatsoever. I tried Disk Utilities option First Aid which resulted in the following message:

Why is that? I never had - and still don't have - any problems with the disk so what could cause this message? Tried it several times but to no avail. I copied several folders with 2 - 5000+ files and deleted them, just to test this feature and it worked. I compared the copied files with the originals and there were no differences. So what is the matter with First aid?

Thanks in advance, guys

Hans