How many Thunderbolt 3 hard drives can be connected to an iMac 2019?

Emm W wrote:

provided of course that I really can daisy-chain all six to one of the iMac TB3 ports (which given the answers below I'm not yet completely sure I can do; someone mentioned a limit of 5 units, someone mentioned up to 6 units)?

The first 5 in my reply was a typo. I meant to say that the limit is 6 per TB host port – including any Thunderbolt docks or hubs that might be hanging off that port.

PS! I will look into the OWC bays mentioned; what I had otherwise found before is this bay from Synology:

https://www.proshop.se/Datalagring/Synology-DX1215II/2973828

This however doesn't seem to have a TB3/USB-C port, but rather something called "InfiniBand", which if I understand correctly should instead be connected to the iMac EtherNet port??

The Synology Web site says that the DX1215II is a "plug-and-play" expansion unit for a Synology DiskStation. I would guess that InfiniBand is some sort of proprietary connection that the DX1215II uses to communicate with compatible DiskStation products. Presumably you would look at specifications for the DiskStations to see how they would work with your Mac.

https://www.synology.com/en-us/products/DX1215II#features

I found an InfiniBand cable on a third-party retail site. Based on the picture and the wording "SATAx4", it doesn't look like any Ethernet cable that could plug into your Mac. It does not even look like a SFP+ or QSFP cable of the sort that might plug into enterprise-level networking equipment.

https://avoira.com/synology-cable-infiniband-infiniband-cable-0-63-m-satax4-black-brushed-steel-cable-infiniband.html

So you'll need to find a DiskStation to go with that drive bay unit …

It's my understanding that a Thunderbolt chain can include up to 5 devices. If you use a Thunderbolt hub (like the OWC Thunderbolt hub) to split one chain into three, it doesn't raise the limit, and the hub itself counts as a device, so then you have a limit of 5 devices attached via three chains.

Of course, there are some Thunderbolt drive enclosures that have room for multiple drives. For instance, OWC has ThunderBay 8 and ThunderBay Flex 8 enclosures that can hold up to 8 drives (if you've got a ton of $$$$$ to burn on the drives and enclosures).

If you daisy-chained six of those to one Thunderbolt port, you could theoretically have up to 48 hard drives hanging off that port. Though I wouldn't advise you to try to access very many of them simultaneously!

The 2019 iMacs have two Thunderbolt 3 ports. I'm guessing that the ports share the same Thunderbolt controller – that there's only 40 Gbps bandwidth (total) for both ports. Not sure if there's a limit of 6 devices (total) for both, or if you can have a 6-device chain hanging off of each. If you could have 12 eight-drive enclosures hanging off your iMac, then you could attach 96 hard drives.

And your office would look and sound more like the machine room of a corporate I.T. department than like a place where you would want to live.

I would not try to daisy chain them to a single port on the back of the iMac. I'd get self powered dock with a number of appropriate ports and use it.

I recommend looking at the various docks at OWC (MacSales.com) as they have proven reliability, excellent customer support and warranties.

For your particular iMac model contact OWC's customer support and get their recommendation for the dockl that would best suit your iMac model, work requirements and budget. 

There is at least one 22 TB hard drive on the market. 96 of those would give you 2,112 TB. Let's say that the hard drives you can actually get can sustain transfer rates of 262 MB/s (2 Gb/s) (based on this article). That sounds way too high to me, and it doesn't account for overhead, but for fun, let's run with it.

Then to read or write all 2,112 TB, your Mac would need to do non-stop disk I/O for almost 100 twenty-four hour days. Even if you had enough money to build a machine room in your house and fill it with 96 hard drives, I don't think you'd have the free time to sit around and use them all!

I have seven attached to my 2015 21.5" iMac.

Four are in a self powered four bay USB HDD "holder". Can't think of the "correct" term for the device.

The other three (SSDs) are attached directly to the Mac.

All work as expected.

:)