Error backup job 'name': destination cannot be accessed. USB Disc. RN316 OS 6.5.0

RN316 F/W OS 6.5.0 (yes I know .1 was just released...)

I have two, same, USB3 external HDDs.

tl;dr - one fails with 'destination cannot be accessed.' when I run a backup to it. It connects OK and I can browse it via 'shares' in Readynas web. It is NTFS, I disconnected it and chkdsk'd it OK. Any ideas? Logs I can check? It completed a backup in the past (full, current job is incremental).

longer vers.

Now that USB3 works properly, I setup my backups.

My plan was to have a father/son offsite backup, alernating each week.

So two backup jobs, which run on a Monday, one will fail as that HDD is not connected.

The backup jobs auto-eject.

So I unplug one when the email tells me it is done. Put it offsite, get the other, plug it in for the next week,when the other job will run.

The two are named Red & Black (case colours) and ReadyNAS calls them USB_HDD_4 & 5.

I started with Red(4), a full backup. OK

Then next week Black(5), again a full backup. OK.

{I also did a test incremental i.e 2nd run, to Black here, OK}

This weekend I plugged Red back in, connected OK, however when the job ran it got the error 'Fail: destination cannot be accessed.' (log below).

Note this would be Red 1st incremental backup, as 'schedule full backup' was set to 'first time'

I re-connected, re-ran, same.

I ejected, chkdsk'd it OK.

Reconnected, re-ran, same. I can browse it OK.

I got Black(5) and ran its backup (again incremental), works OK.

The two jobs are identical except name and destination.

Any ideas?

Note, on checking kernal.log, I note (was reminded) Red was formatted in Windows 7, I can't remember formatting Black, may have been out-of-box.

I cut-down and annotated the log here.