Reddit reviews StarTech.com 2 Port SATA 6 Gbps PCI Express eSATA Controller Card - Storage Controller - 2 Channel - eSATA 6Gb/s - 6 Gbit/s - PCIe - PEXESAT32

I'm surprised that you haven't gotten much response in the last day. I suppose the FAQ is there, but you've looked through it and it doesn't provide you with the answers you're looking for so you're asking the community. So... what to do next? I'm gonna assume you're asking about what to do next hardware wise. Something that is low cost and won't lock you into something you can't change later.
40-50TB is nothing to scoff at, I'm surprised you've managed to get by with hooking them up direct this long, but hey if it works, it works (it's not dumb if it works).
A baby step could be increasing your density.
Grabbing a MediaSonic Pro Box https://www.amazon.com/Mediasonic-ProBox-HF2-SU3S2-SATA-Enclosure/dp/B003X26VV4/ref=sr_1_3?ie=UTF8&qid=1540336320&sr=8-3 and shucking your externals into a 4up bay would free up some ports.
Then a cheap next step would be to upgrade from USB3 to some ESATA ports. I use this ESATA card https://www.amazon.com/StarTech-Port-Express-eSATA-Controller/dp/B003GSGMPU/ref=sr_1_2?ie=UTF8&qid=1540336457&sr=8-2 to hook up to the MediaSonic box and it works great. I still run this setup on one of the computers in my garage for "offline" backups. I just WOL the machine and backup some critical data then power it down.
If you want to start offloading the workload to another computer all this stuff would transfer well into another computer.

Other than that the FAQ does have a ton of good recommendations for what software to run under each different platform WINDOWS/MAC/Linux. In this day in age I don't think there is a "wrong" choice. Work in what you're most comfortable in. I'm partial to Windows and then Linux (mainly Ubuntu). If you have any other questions feel free to reach out.

Cool ended up with these 2.


https://www.amazon.com/gp/product/B00N1Q2M6A/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B003GSGMPU/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Taking your advice I decided to do the following:

Order this card and use software raid 5 for redundancy. This will require me to purchase another 8TB drive but I think the benefits out weigh the cost. Thus giving me 3x8TB in raid 5.

Thank you for all the help and information pertaining this issue. Reddit to the rescue yet again!

Current: (6-1) x 4 TB = 20 TB

New:
(3-1) x 6 TB = 12 TB
(3-1) x 4 TB = 8 TB
20 TB total

You don't gain any space by doing this, though you do prepare for the future.

Are you able to add more drives to your system, perhaps externally? I've personally used these Mediasonic 4-bay enclosures along with an eSATA controller (though the enclosures also support USB3). Get some black electrical tape though, because the blue lights on the enclosure are brighter than the sun. The only downside with port-splitter enclosures is that if one drive fails and knocks out the SATA bus, the other 3 drives will drop offline too. The infamous 3 TB Seagates did that, but I had other drives (both 3 TB WD and 2 TB Seagates) fail without interfering with the other drives. Nothing was permanently damaged; just had to remove the failed drive before the other 3 started working again. Also, the enclosure is not hot-swap; you have to power down to replace drives. But hey, it's $99 for 4 drive bays.

6 TB Red drives are $200 right now ($33/TB); 8 TB are $250 ($31/TB), and 10 TB are $279 ($28/TB).

Instead of spending $600 (three 6 TB drives) and getting nothing, spend $672 ($558 for two 10 TB drives, $100 for enclosure, $30 for controller, $4 for black electrical tape) and get +10 TB by adding a pair of 10 TB drives in a mirror in an enclosure, and have another 2 bays free for future expansion.

(6-1) x 4 TB = 20 TB
(2-1) x 10 TB = 10 TB
30 TB total, $668 for +10 TB

Later buy another two 10 TB drives and put them in the two empty slots:

(6-1) x 4 TB = 20 TB
(2-1) x 10 TB = 10 TB
(2-1) x 10 TB = 10 TB
40 TB total, $558 for +10 TB

Then in the future you only have to upgrade two drives at a time, and you can replace your smallest drives with the now-replaced drives.

You can repeat this with a second enclosure, of course. :)

Don't forget that some of your drives will fail outside of warranty, which can speed your replacement plans. If a 4 TB drive fails, go ahead and replace it with a 10 TB drive. You won't see any immediate effect, but you'll turn that 20 TB RAIDz1 into 50 TB that much quicker.

Oh, and make sure you've set your recordsize to save some space! For datasets where you're mainly storing large video files, set your recordsize to 1 MB: "zfs set recordsize=1M poolname/datasetname". This only takes effect on new writes, so you'd have to re-write your existing files to see any difference. You can rewrite files with "cp -a filename tmpfile; mv tmpfile filename" for all files, or a much easier way is just create a new dataset with the proper recordsize, move all files over, then delete the old dataset and rename the new dataset.

See this spreadsheet. With 6 disks in RAIDz1 and the default 128K record size (16 sectors on the chart) you're losing 20% to parity. With 1M record size (256 sectors on the chart) you're losing only 17% to parity. 3% for free!

https://www.reddit.com/r/zfs/comments/9pawl7/zfs_space_efficiency_and_performance_comparison/
https://www.reddit.com/r/zfs/comments/b931o0/zfs_recordsize_faq/

That’s interesting... I got this: StarTech PEXESAT32 2 Port SATA 6 Gbps PCI Express eSATA Controller Card https://www.amazon.com/dp/B003GSGMPU Has been working great and haven’t noticed a slowdown.

https://www.amazon.com/StarTech-PEXESAT32-Express-eSATA-Controller/dp/B003GSGMPU/ref=sr_1_3?s=electronics&ie=UTF8&qid=1540578064&sr=1-3&keywords=esata+card

Thanks, any particular ones you'd recommend?

You mean something like this, right? https://www.amazon.com/Mediasonic-ProBox-HF2-SU3S2-SATA-Enclosure/dp/B003X26VV4

and then this for the card? https://www.amazon.com/StarTech-PEXESAT32-Express-eSATA-Controller/dp/B003GSGMPU/