Umfriend

I am assuming no duplication, no backups and stable connections to the hard drives. In that case, I would think a single large Pool makes most sense. The reason being that should scanner find an issue and trigger DP to evacuate a drive, you have the best chances the other drives will be able to take on the load. With smaller pools, that is a question for each pool separately.
Performance wise? No issue or noticeable difference.

I am assuming no duplication, no backups and stable connections to the hard drives. In that case, I would think a single large Pool makes most sense. The reason being that should scanner find an issue and trigger DP to evacuate a drive, you have the best chances the other drives will be able to take on the load. With smaller pools, that is a question for each pool separately.
Performance wise? No issue or noticeable difference.

I am assuming no duplication, no backups and stable connections to the hard drives. In that case, I would think a single large Pool makes most sense. The reason being that should scanner find an issue and trigger DP to evacuate a drive, you have the best chances the other drives will be able to take on the load. With smaller pools, that is a question for each pool separately.
Performance wise? No issue or noticeable difference.

In the DP GUI, see the two arrows to the right of the balancing status bar? If you press that, it will increase the I/O priority of DP. May help some. Other that that, ouch! Those are more like SMR-speeds.

That is correct.
And you can have files outside the Pool on a Pooled drive as well, like the lightroom folder. No clue why anyone would want that but...you can.

So AFAIK, everything DP reports wrt space is in terms of actual disk space. Assume a Pool of 2 x 10TB drives, x2 duplication except for one folder named UDF (UnDuplicated Files).
- Pool is empty and shows 20TB free
- Save a 1TB file to the Pool. The Pool will show 2TB Duplicated and 18TB free.
- Save 2 100GB files to UDF (two files because one would complicate a bit and cause Unusable for duplication, now one is on one drive, the other on the other). Pool will show 2TB duplication, 0.2TB unduplicated and 17.8TB free.
A duplicated file is indeed seen as a single file _assuming_ you access the file through the Pool / DP. However, a x2 duplicated file is stored on two HDDs. Each has a hidden PoolPart.* folder within which the file resides. If you access the HDDs directly, you can see two files. In the above example, if you pulled one HDD (so it is still working) and put it into another PC, then yes, on that PC you can simply navigate to the PoolPart.* folder and find all your, duplicated, files (unduplicated is a matter of chance of course). DP stores everything in plain NTFS so that is nice from a recovery point of view.
Now assume one of the 10TB drives crashes, hard. DP will put the Pool in read-only mode. You will add a new 10TB HDD to the Pool and remove (through the GUI) the faulty one. At that time, DP will check for duplication and find none are duplicated even though most should be. DP will then re-duplicate. Non-duplicated files stored on the faulty HDD are lost (barring recovery and/or actual backups).
I guess that's basically it.

No, you only need a license on Server and then allow PC and laptop to access a share that points to the virtual disk DP makes for you.
On Android, no clue.

I understand the concern.
I use DP with x2 duplication and Windows Server Backup and that indeed relies on VSS. So what I have done is I created two Pool (Q:\ and R:\) with x1 (so no) duplication and then created another Pool P:\ that consists of Pools Q:\ and R:\ and Pool P:\ does have x2 duplication. This way I have the best of both worlds I guess, I got x2 duplication and x1 backup.
It would work similarly with x3 duplication, i.e., create three Pools without and then add them to a Pool with x3 duplication etc. It is not without its compromises, you would want, for instance, each underlying Pool to be roughly the same size and as if and when a disk in the Pool that is being backed up fails, you may need to review the backup settings (as it may need to point to a replacement disk).

Awesome! Yes, install that drive, add it to the Pool, click on remove for the faulty drive and let DP do its magic. It may take quite some time but, especially as I think you do not have duplication, you want to get the Pool healthy ASAP.
The faulty drive can be tested, there are various ways and tools to do so. If you can RMA, then that is the way to go, if not you can try that. In and by itselfm a few damaged sectors is no issue (I have them one one HDD due to a power failure). However, if you can get the number of damaged sectors to increase then I would not use it at all other than to experiment with disaster recovery
On permissions, yes, that is rather common I think. Do this http://wiki.covecube.com/StableBit_DrivePool_Q5510455. Personally, I would do this after restoring the Pool to health.

There is no database in DP so if files were lost there is no telling which (unless you have a file listing / database yourself somewhere). However, no files that were duplicated should be lost.

On the two HDDs, there are now two hidden poolpart.* folders, one within the other. What you would do is:
1. Stop DrivePool service
2. On each HDD itself, move the contents from one poolpart.* folder to the other
3. Start DrivePool service
4. Remeasure / Rebalance. I think this will start by itself.
The full howto from Stablebit is here: http://wiki.covecube.com/StableBit_DrivePool_Q4142489