What causes FCPX to fail when importing files on Mac?

Started as a FCPX issue. Was importing files from SD cards and it wouldn't do it. Thought there was an issue with the original media. But wasn't in the end. Seems I cannot write to a 3 month old Crucial SSD drive formatted APFS volume.

Was working ok yesterday and completed a few edits with this drive. Not I cannot copy a single file to it: Error attached. Disk Ultility First Aid won't fix it either. Previously said something about Space Manager. Now it's saying:

Running First Aid on “FCPX SSD 1” (disk5s1)

Checking file system and repairing if necessary and if possible.

Volume could not be unmounted.

Restoring the original state found as mounted.

The repair cannot be performed because one or more volumes are mounted. : (-69565)

Operation failed…

Total PITA as I have a bunch of stuff needing done now. The drive is virtually new (4TB) and has about 150gb on it now. Any suggestions? Is there anything I am doing wrong with Disk First Aid? Next steps??

Thanks in advance.

[Re-Titled by Moderator]