External Hard Drive won't mount in Sequoia 15.3

To be sure the hub is not an issue, connect the HD directly to the computer and if it mounts, then you know the hub unpowered is not going to work. Disconnect all other USB devices for the test. Myself and others have found the OS is not the issue. Another good test is to connect your external HD to a computer you know is functioning OK with another external USB HD.

OOPs, meant...

Open Activity Monitor in Utilities, View>All Processes, sort on Name, see if fsck is running, if so force quit it.

Open activity choose show all processes, see if fsck is running.

I have two external USB HD and both mount on my macStudio M1 running 15.3.1 and both drives are pushing ten years old, Seagate and a WD. Another drive, OWC connects thru thunderbolt with no difficulty. So the OS would seem ok and important to access a computer known to be OK to check the HD. Or borrow a friend's working HD and try on your computer. Another test is to mount a flash drive, not the same situation as an external HD but verifies the computer will recognize an external memory device.

The definitive way to know the condition of your external HD is to take the "non-working" drive and connect to a known working computer to verify whether they will mount or not. Cables, hub, overloaded USB line, as well as hardware issues may be at play in your situations, the OS not likely. Once I had a newly purchased drive that gave erratic behavior and calling the manufacturer produced a prospect of long troubleshooting, so then I returned it to where I bought it for refund and purchased a different one and all was well. I was confident my computer was functioning ok since no trouble signs had occurred before the new drive. Age alone is not a compelling way to make decisions as often there are warning signs like in reading or writing difficulties or slow mounting. Call the manufacturer or your store about the new SSD and see what they advise, or return it and purchase another.

I would try mounting it in safe mode and see if it mounts.

Also, boot from recovery mode and see if you can use disk first aid on the drive.

And, are these drives Bus powered or do they have their own power?

System Settings>General>About, scroll down, System Report>Hardware>USB, what mA are used & available?

Just to be sure, would you connect this HD to another computer and verify it mounts please?

My setup is similar to yours, MacStudioM1 with an external HD (OWC-three years old) and have no issue with the external HD and did the 15.3 update successfully. Might you be able to connect that HD to a different computer just to verify it does function ok?

If a USB3 drive then a powered USB3 hub will do it hooked to your USB3 bus on your Mac.

I strongly suggest AC powered USB hubs.

I have M1 Mac with 15.3.2 and a usb disk will not mount. It is powered by the computer not a plug in.

Tried all disk utility on two machines both running 15.3. Other disk are acting up as well. This occured after update to 15.3.2. I have 9 drives will this be an issue for the remainder of my time with 15.3

If your purchase has been recent, like maybe 30 days, check your receipt, stores will take back no questions asked in many cases. It's ok to just say it stopped working. If too long, the HD has a warranty from the manufacturer and contacting them will lead to a discussion and be asked to do some checks. Hopefully it leads to a solution or they offer to send a free replacement. Warranty typically are at least two or more years, calling they will let you know. You should have the receipt. And if you have problems, you have done things and have strange results, and believe the computer is the issue (usually not with HD situations) call AppleCare 800-275-2273 and they will help even when out of warranty.

Are you saying it mounted when at 15.2.x ? or have you just discovered it doesn't mount? Make sure the connections are correct.