Files "disappear" from a mounted external hard drive

Boot into Safe Mode by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

Hello there Kiwi in PA,

We'd like to see how we can help with your external hard drive and your iMac.

First, we'd like to ask a few questions to gain a better understanding of your issue. Do you use any other drives? If so, do they also experience the same issue? Have you attempted to try different USB ports on your Mac? Do other USB devices experience issues when connected to your iMac?

With that info, we can develop a clear path to getting this resolved.

Cheers.

Thanks for following up with us,

We'd like to see how we can isolate this issue down to help us clear a path to resolving this issue.

We'd like to see if the issue is persistent in a new user account. You can do so by going to System Preferences > Users & Groups. Once in a new user account, we'd like for you to see if the behavior continues. With that info, we can determine if the issue is user based, or system-wide.

We look forward to your findings.

Cheers.

Hey there,

Thanks for following up with us. We're committed to getting this issue resolved for you.

We'd like to continue isolating the issue down to a specific component or software. Our next step would be to have you, if possible, test these drives on a different computer, Mac or PC, to see if this behavior continues.

If you don't have the option to test out the drive on a different computer, we'd like to have you reach out to our support team using this link: Get Support

We look forward to continue to working with you.

Cheers.

We appreciate you reaching your follow up,

We would recommend at this point reaching out to our support team to continue troubleshooting and investigating this issue for you. You can following this link to get in touch with them: Get Support

We look forward to continue working with you.

Cheers.