Crucial X9 Pro for Mac

I/O Errors are usually caused by some sort of hardware issue. And those I/O errors are likely corrupting the file system as well so you probably now have two issues with that external drive. There is no sense attempting file system repairs until you address the hardware issue.

As others have suggested, the USB cable is the easiest first step.

You could also have an issue with the USB-C port on the computer. Make sure to connect the SSD directly to the computer since adapters, docks, hubs may introduce problems. If the SSD is connected directly to the computer, then try using a USB3 hub as I've seen some reports on this forum that sometimes this will fix those disconnect issues even if the hub is not a powered hub (weird I know). Also, try disconnecting all other external devices from the computer just in case one of them is causing a problem & compatibility issue even if the SSD is not directly connected to those other devices. It is also possible the USB port on the external SSD has a problem, or perhaps the whole external SSD has a problem.

You can try checking the health of the external drive by using DriveDx (free trial period). Post the complete DriveDx text report here using the "Additional Text" icon which looks like a piece of paper on the forum editing toolbar. Unfortunately an SSD's health report needs to be manually examined & interpreted since some "failing" or "warning" notices may not really be an immediate problem. A special USB driver will need to be installed to check the health of an external drive. I'm not certain, but there may be an issue with that special USB driver when it comes to uninstalling it due to macOS security features or a bug when used on macOS Sequoia (I cannot find the reference to it at this time to confirm). If you have access to an older Mac, then you should have no issues uninstalling the special USB driver. Or if you have a Windows computer, you can use GSmartControl to check the SSD's health (again, post the complete report here for examination). Windows File Explorer will not see any mountable volumes which is normal since your drive is using the APFS file system, Just don't erase the drive. You can still use these utilities to check the health of the drive as long as the physical drive is seen by the system (you may not see it in Finder or File Explorer...doesn't matter).

Since you are using an M-series Mac, you can try booting into Safe Mode to see if you encounter the same issue. However, you won't be able to mount the external drive if it is using the NTFS file system (more for other users with similar issues regarding a data only drive). Safe Mode may not be an option for Intel Mac users since macOS seems to disable so many drivers at least on later model Macs & macOS for Intel Macs that even external drives may not be available (only discovered this "feature" last week).

Have you added any other USB devices to the iMac..?

If so, that device could be causing the X9 to lose power and eject.

1) Try disconnecting any other non-essential USB devices.

2) Try moving the X9 to a different port.

3) Try a different USB-C cable.

The Crucial X9's Enclosure Chip or SSD could also be failing and the drive needs to be replaced.

https://www.crucial.com/ssd/x9-pro/CT1000X9PROSSD9?

Whenever you get the “improperly ejected” message the very first thing to try is making sure the cable is completely inserted in the ports (both ends), then try using a different cable. Nine times out of ten that’s the problem.

Second Ikrupp's suggestion on the cable.

Quite a few years ago now, but I had an external drive connected with a FireWire 800 cable. The drive literally never moved from its spot on the desk, so neither did the cable. And yet after a couple of years, I started getting constant disconnect error messages. The last thing I suspected was the cable since, how could something that never so much as gets touched be the problem? But as soon as I replaced the cable, the problem was gone.

I am using the X9 and X9 Pro without incident on two different Macs running Sequoia v15.5 as APFS formatted Time Machine drives:

• M4 Mac Mini Pro
• M1 Mac Book Pro (16-in)

I am also using an X9 1TB (FAT32) with my ripped CD collection in my 2011 Saab 9-5 via the AUX input with a USB to USB-C adapter. Works perfectly and tracks change instantly.

In all cases, I use the USB-C 3.2 cable that came with the drive.

In your circumstance, I would check the cable connection and then boot the Mac into Recovery mode to run Disk Utility First Aid on the drive. Sometimes gremlins get in there…