Edward

Just to wrap this thread up. Evacuated failing drive and reduped all data. 
Essentially I followed the recommended DP process of removing a drive, adding a new drive and letting DP do its stuff.  That went smoothly
I figured that as I anyway had a backup of the failing drive poolpart folder to cover me as a plan b as it were I may as well just try the recommended process. I was surprised how fast the evacuation took (only about an hour).  Previously (some years ago) the evacuation of a failing drive never completed (after running for days) which required me to just pull the drive.  This made me cautious leading me to take a copy of poolpart whilst I still had access to the failing drive.  Luckily this time the failing drive stayed up long enough to complete an evacuation. 
One quirk I did find was when I tried to add the drive to the pool that contained the copy of the poolpart folder.  This drive was never used in the pool before, however it errored with "Cannot add the same disk to the pool twice".  Clearly this was nonsense (as the drive was never used in the pool even once) but clearly it was due to the poolpart folder name. I simply renamed the poolpart folder which allowed DP to go through a measure and redup process. I then deleted the backup poolpart folder.
So many thanks to @Jonibhoni and @marquis6461 for their kind help. 

Christopher (at support) came back on my ticket.  He reenabled my ID and all now good.  Can write to the DrivePool.
Just in case someone else encounters this problem use a fresh trial ID to get things going while waiting for support to reenable the paid ID. 

Thanks guys
I plucked up the courage and went to beta on all apps and connected to the new cloud thing.  Looking good!

Thanks guys
I plucked up the courage and went to beta on all apps and connected to the new cloud thing.  Looking good!

Same here. The bonus with DrivePool is that if it fails you don't lose data, just the pooled drive itself. Worst come to worst, just reinstall the earlier version again.

been on DP beta all the time for years - no issues of note

yep - beta apps
new ones posted today

Fully recognize that the current issue is not mine (but I'm the OP) however would highly appreciate if:
1. How do I find out which files on drives are unduplicated?
2.  That this thread is anyway updated with recommended processes/commands need to be followed when a problem occurs.  Or a link to such processes/commands.
cheers
Edward

Once you've created the nested pool, you'll need to move all of the existing data into the poolpart hidden folder within the outer poolpart hidden folder before it will be accessible from the pool. It's the same process that you need to complete if you simply added a drive to a non-nested pool that already had data on it. If you want the data to be accessible within the pool, you'll have to move the data into the pool structure. Right now you should have drives with a hidden poolpart folder and all of the other data on the drive within your subpool. You need to take all of that other data and simply move it within the hidden folder. See this older thread for a similar situation: https://community.covecube.com/index.php?/topic/4040-data-now-showing-in-hierarchical-pool/&sortby=date
 

Re long names.  Luckily the culprits were in one folder so I simply moved the folder to the root, renamed the offending files and moved the folder back again.
 
Yeah WinDirStat is one of the tools in my toolbox. Simple, clean, fast.
 
Re Windows 10.  I have a Win10 new bare metal install already done, just waiting for you guys to give the green light and I will move my data drives over (thus replacing the WHS11 instance I have been using for ages).
 
Edward

Thanks for the feedback Christopher.  As always great feedback and good transparency.  A model that other businesses could well emulate and benefit from.
 
My state of play now is that my pool is smaller (due to the pulled drive) and everything looks clean with all folders etc duplicated (save for some annoying long name files which I can't even kill or rename in DOS).
 
New WD Red 6tb drive is currently being scanned and once it comes up clean overnight I will add to the pool. Thanks for your tip about possible infant mortality.
 
I'm interested to see how the duplication balancing works out given the significant difference in drive sizes (pool will be comprised of 2*2tb, 1*3tb and 1*6tb). The pool will need to survive the 6tb drive going south.
 
One good thing about all of this is I discovered an old temporary backup of something which was about 900gb which I no longer need and once I killed that my pool free space increased by 1.8tb. The not needed files were lurking deep in a folder somewhere. <oops>.
 
 
 
Edward