Andre - Splitting ZFS array

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I started my FreeNAS experience with version 9.3 and used the upgrades till now, However after upgrading to 11.1 a "New feature flags are available for volume rpool" warning appeared. Based on Allans advise I will be migrating to two 3 disk RAID-Z1s with 4Tb and 6Tb WD reds and not upgrade my current pool with the new feature flags. Also, I found a great deal for a Supermicro X10sdv-4C-TLN2f (€100) and am planning to do a fresh FreeBSB 11.2 install. Currently I have my drives 4x4Tb and 1x6Tb in one 5 disk RAID-Z1. Now I would like to reuse my 6Tb red with two new drives for the start of the new pool, however if I remove the drive from the 5 disk pool before the data is migrated would put my data at risk (I have backups of my importand data, but not of my music and video's which I could re rip). My 16Tb pool is currently 78% filled making a full backup difficult. I have been reading up on some "hacks", but do not feel confident about using them.
The first option would be to buy 6Tb drives and put them in a degraded 3 disk RAD-Z1, Migrade the bulk of the data and than resilver with the old 6Tb drive.
My second idea would be to create a 6Tb stripe out of smaller drives and create a temporary 3x6Tb RAID-Z1 with the two new drives.
My third option would be to bite the bullet and just buy 3 drives, however I am hoping I could save that money and spent in later on to replace 4Tb drives with 10Tb versions when prices drop.
An other option would be to start my new pool with 2Tb (Samsung F4) drives and move my files to the new pool more slowly.
This last option triggered a question witch is probably quit obvious to most listeners/viewers but I have not been able to find a definitive answer. I know you can force ashift=12 on 512k drives but how do I know for sure my pool is written with 4k sectors? The main reason I started doubting the facts are that HGST is offering both 512 and 4k drives in most of their sizes and WD does not as far as I know. Would a emulated 512 drive split up the 4k sectors that are offered or does ZFS overrule this.

My core question is, what does ZFS do with HDD sector sizes and emulation.

Thanks a lot and keep up the great show.

Kind regards,
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