The answer that worked for me is what drmwho noted above:
<quote>
I found some suggestions on the LaCie support site.
Some Seagate and LaCie software use a kernel extension (kext). Apple
silicon Macs require that users change the security settings to ‘Reduced
Security’ for the software to function properly.
To change the level of security on your startup disk:
Shut down the Mac
Press and hold the power button until you see “Loading startup options”
Click Options
Click Continue
If asked, select a user > Click Next > Enter password for admin account
In the menu bar at the top, Select Utilities > Startup Security Utility
Select the startup disk
If the disk is encrypted, Click Unlock > Enter Password > Click Unlock
Click Security Policy
Select Reduced Security
Put a check next to “Allow user management of kernel extensions from identified developers”
Click OK
Enter admin password > click OK
Once changes are applied, click the Apple icon and Restart
Once you adjust startup security, restart and reconnect the drive, it should work. It has to do with letting the drive's software install a kext so it can mount, as I understand it.
Good Luck! I struggled and cursed Apple's lack of info on this, finally succeeded.
I also found Glenn Fleishman's "Take Control of Your M-Series Mac" very helpful, although he does not specifically address this as a fix for hard drives not mounting. His section on the startup security settings gave me confidence that this was a real, safe thing to do (not just a drive manufacturer hack). Lots of other great M1 info also.
Once I did this, the drive mounted and showed up in Disk Utility, which it would not do before. Hooray!
Solved ... for me at least ....Disk Utility -> First Aid
I had a very similar issue with my brand new Macbook Air M1 with BigSur 11.4
I have two Seagate 8TB drives, both formatted as eXFAT, which worked perfectly on my 2015 Macbook Pro Retina and an aged Dell M6800 running Windows 10.
No matter which drive, no matter how connected (Anker 6-in-1 PowerExpand usb-c hub. or another dongle usb-c to usb-3), I would ... a) not see the drive in Finder b) In Disk Utility the drive would be seen as not mounted, and would not mount manually c) powering up with the drive made no difference
Following an Apple Tip in this thread, I then ran First Aid on the drive using Disk Utility, but... FIRST: attempt a Disk Utility -> Mount, wait a while and close the error message that says can't mount .... important step. Then Run First Aid ...... a) select the show details tick box so you can see what's going on then proceed b) Accept any "can't mount" or "unmount" messages if they appear c) after very few minutes the drive was "repaired", mounted, visible in Finder, and all files & folders seemed intact. d) continued to work when drive disconnected & reconnected Most importantly, after the "repair" the drive still worked perfectly on my old Macbook & PC
I'll check out that the drive works with the other dongles and also do the same on my other 8TB drive.
I've just done that with the same results.
I agree 100%. What is wrong with this company? They freely make changes that affects our data and hardware and then ignore us, the customers, with all the problems their changes create. And it's not; like they don't have the money to support their customers.
Apple killed its desktop unit a few years ago. Alas. latest OS's show the loss.