Had run out of hard drive memory on my 24", 2021 iMac M1 running Sequoia 15.3.2 so I created a boot drive on a Samsung T7 with 2TB. So far so good. I did discover that of the measly 2 USB-C ports (the only ports on this model, besides a stereo miniplug) only one would support a boot drive. A week or so after setting this system up I wanted to scan a few pages with my Epson Perfection V550 Photo (no Wi-Fi or Bluetooth). It had worked fine while booting from the 245 GB internal HD on the iMac by connecting the scanner to a USB 2.0 10-Port Hub and the Hub to the second iMac port via an Anker USB-C to USB3.0 Female Adapter for the Hub USB cable. This arrangement no longer works and the scanner is not found either when connecting the scanner directly to the second iMac USB-C port via the Anker adapter. Am I missing something?
With the scanner attached, run a System Profile from ‘About this Mac’, and go to the USB and thunderbolt sections. Somewhere in the tree you should at least see the entry for the 2.0 hub, and hopefully all the devices on the hub.
If not, disconnect the scanner, and scan again (⌘ - R ) for the hub. If not present, your hub might’ve died; if present, your scanner may have died. Plug some other devices to test. Also try different ports on the hub; I am still using an 11 port USB-A 2.0 hub with three dead ports. Just pt tape over the bad ports and move along.
HTH
PS: is it a powered hub? Maybe the USB-C port on the Mac is not providing enough power for the attached devices. Can you remove the hub from the equation, testing only if the scanner can work directly on the Mac.
This is a reply to both your comments, frederico, thank you for the thoughtful responses.
Sorry I did not mention it was a powered hub and its light is on. Since I posted my question it did occur to me that connecting the scanner cable with the Anker USB-A to USB-C adapter directly to the USB-C iMac port might work, I did. The scanner chortled and then showed up in the Devices list of the Finder Window. I was also able to read my two older external disc hard drives with 1TB and 2 TB via direct connection to the iMac port. This all occurred after moving the boot drive to a Samsung 2TB T7. None of the ports of the hub allowed the 3 devices I’ve mentioned access to the boot drive. I have looked for a purely USB-C hub, one that has only USB-C inputs and outputs, unsuccessfully. Has anyone seen such a thing?
Absolutely. If it’s just about 10 Gbps USB-C (and not TB) they’re quite inexpensive with price usually depending just on number/type of ports and PD wattage.
Since you have an iMac, you don’t need to worry about the hub’s ability to charge the upstream host.
Cheaper variants usually support only data, more expensive models support video through USB’s DP alt mode.
Here are a couple decent examples.
https://www.amazon.com/dp/B0B3J2JS1Q/
https://www.amazon.com/UGREEN-Adapter-Multiport-Splitter-MacBook/dp/B0C4DFB8RH/
https://www.amazon.com/Splitter-Multiport-Adapter-MacBook-Chromebook/dp/B0CYLPVN4B/
https://www.amazon.com/dp/B0DMDZR7FZ/
Thanks, Simon, that’s the kind of information I’ve been looking for. Do you expect these hubs will allow the Epson scanner and 2 external hard drives to be readable by my iMac due to differences in the USB-C hubs’ specs? I am not knowledgable enough to figure this.
If this were all just about flash memory (SSDs), I’d say it wouldn’t matter. But if you want to attach HDDs and that old scanner, I’d make sure to get a powered hub, i.e. with support for 100W DP. If the devices don’t work straight away attached to the hub (or the disks appear flakey connected to it), you just plug that hub into a USB charger and then everything should be powered fine (i.e. at the 15-W spec for USB-C).
Thanks, again, Simon. The old scanner is perfectly fine however and isn’t ready for the dust heap yet. 
To rf806, how did you create an SSD boot drive? I have a mac mini M2 with the latest Sequoia and have tried to make a boot drive with Apples Disk Utility, Carbon Copy Cloner, and Super Duper, all of which are supposed to create bootable copies. None of the copies worked. They all got stuck in a loop trying to boot and shutting down, rinse and repeat.
I there a magic incantation? My internal SSD is practically full.
… None of which make that claim anymore. CCC and SD both state that they use Apple’s internal utility, which doesn’t always work reliably.
The only official way to make a bootable SSD is to run an Apple installer. For a modern Apple Silicon Mac, this pretty much means making a bootable installer (onto a thumb drive), boot that (which will bring you to the installer’s Recovery mode) and from there run the macOS installer to install/upgrade macOS on your external drive.
Are these devices bus-powered? If so, then the hub needs to be able to supply sufficient current to all the ports at once. Some can’t handle it. And some come with a power adapter that isn’t big enough to simultaneously power multiple high-current devices.
But if the devices have their own power supplies (as I would expect from a scanner or a 3.5" HDD), then it really shouldn’t matter, although that’s no guarantee.
The presence of type-A or type-C connectors shouldn’t matter. USB 3 permits up to 900 mA (4.5 W) per port (up to 6 units of 150 mA each) and USB 2 permits up to 500 mA (2.5W) per port (up to 5 units of 100 mA each). But this is for a powered hub that is capable of supplying that much power. If you have (for example) a 10-port USB 3 hub, then it could be called on to deliver up to 45W (if all ports are drawing maximum power), and it won’t be able to do so unless its power supply is at least 45W (plus losses, so probably closer to 60W in practice). If it only has (for example) a 30W power supply, then some ports are not going to be able to get all the power they need, leading to devices not working or acting flaky.
And if you have some devices that draw more than the standard 2.5/4.5W of power (e.g. via the power-delivery or battery-charging spec), then the hub must support that spec on the port where the device is connected (some hubs only support high power on one or two ports) and its power supply must be large enough to accommodate the total load of all the connected devices.
To gilken, sorry for the delay but I’ve been trying to install OS 15.4 on the Samsung T7 boot drive. The installation failed twice and I have not gotten any hits on my question about this in Apple Communities. I’m not sure if it is my set up or what that is the problem but I will give you this as a caveat before you decide to attempt this. My recollection is that I opened the iMac with the original internal SSD hard drive, opened Disk Utility and Erased the T7 SSD. I then followed the advice in Apple Support titled: Use macOS Recovery on a Mac with Apple silicon. I have found Carbon Copy Cloner to offer useful information as well for certain problems. I’m not at all familiar with the features of mac minis. What I’ve described above may not be relevant for your gear. Good luck. If I figure out how to install the new OS I will post the solution.
To gilken, I just found numerous complaints in Apple Support Community about difficulties preventing installation of OS 15.4 on external drives and internal drives with multiple volumes. Apple will need to offer an answer or ideally a solution.
Thank you shamino and rf806 for your answers. I will try the trick with a bootable installer on a thumb drive to create a copy of the Macs internal system to an external SSD.
To Shamino, thanks for the detailed info on hubs. The truth is I bought a powered USB 2.0 hub randomly years ago for an older iMac, which had USB-A ports, and everything worked fine. Then I bought the current 2021 iMac M1 and needed an USB-A to C adapter for the hub to link my devices with the current iMac and that was fine. But with the newer OS and/or the Samsung T7 being used as the boot drive this set up is not working. I will learn more about hubs and know where I can get answers if they arise.
Definitely take a look at the capacity of the hub’s power brick to see if it’s enough for your devices. I suspect that’s going to have a lot more to do with it than the connector used to attach it to the Mac.
Also, if you’re using a USB 2.0 hub with a storage device like an SSD, you’re completely crippling its performance.
To Shamino, thanks for your suggestions. The 2.0 usb hub worked fine for all my devices until I made the Samsung T7 my boot drive which is connected directly into one of the 2 USB-C PORTS (not hubs) on the iMac. It was the time period after that change when the hub connected scanner and old disc HDs were not showing up on the boot drive screen. I’m aware in general of the differences in data transmission rates as you mention. I’ve used the old disc HD as back up discs via the hub since the back up was taking place during a time of the circadian cycle where it did not limit my access to the iMac.