ZFS: How permanent are Permanent Errors on a single drive?

: Hi Chris, hi Alan
Longtime listener, first call for help.
I've run into some issues on my freenas, which made me wonder about the messages I got.
This boils down to using some older worn down USB flash devices for the most part.
But here is the thing I still struggle with:
Scrub tells me:
scan: scrub repaired 0 in 0h6m with 10 errors on Sun Jul  3 00:44:13 2016
config:
        NAME                                          STATE     READ WRITE CKSUM
        freenas-boot                                  DEGRADED     0     0    20
          gptid/ef389232-4086-11e6-a65c-3464a99ad398  DEGRADED     0     0    60  too many errors

errors: Permanent errors have been detected in the following files:
        //root/FreeNAS-9.10-STABLE-201606270534.iso

I copied this ISO to the root dir on the USB flash devices, just as a test and ran scrub afterwards.

This is a single USB device with no parity/redundancy, so "Permanent errors have been detected" to me means: Sorry, got to let you know, I can not fix this, you better delete the file.
Metadata-blocks could be recovered from the second copy on the device, but data-blocks can not, right?

Is it possible that a sha256 FreeNAS-9.10-STABLE-201606270534.iso returns the original hash at this point?

Am I getting something wrong?

Thanks for the great show all these years!

Cheers
Matt