HOW TO FIX ON MY IMAC The operation can’t be completed because an unexpected error occurred (error code 0).

FYI, there is a big difference between memory & storage.

https://www.crucial.com/articles/about-memory/what-is-the-difference-between-memory-and-storage

If you have completely run out of free storage space on an APFS volume, then you may not be able to free up any space on it unless you can delete an APFS snapshot. If you cannot delete an APFS snapshot to free up storage space, then you will likely need to start over by erasing the drive, reinstalling macOS & restoring from a backup (this destroys data if you don't have a good backup).

View APFS snapshots in Disk Utility on Mac - Apple Support

https://derflounder.wordpress.com/2018/04/07/reclaiming-drive-space-by-thinning-apple-file-system-snapshot-backups/

If you need to recover data from it, then you can try manually mounting it read-only by using the command line.

You should always have at least 20GB+ of free storage space available at all times for the normal operation of macOS. Many workloads require you to keep even more free space available at all times. Keep in mind 20GB can disappear very quickly during normal use due to the use of temp & cache files used by apps & macOS.

You should always have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.

Possible hard drive failure in progress! ⚠️

Based on your situation, I recommend service.

Service Checklist:

• Mac service pricing.
• Check AppleCare Coverage.
• Get your Mac ready for service.

Click ▶︎ Start a Service Request. ⚛️

Need more help?

• Call Apple Support at 1-800-APLCARE. 📞
• International Support. 🌐
• Contact Apple Support. 📝
• Live chat using Messages. 💬

Click ▶︎ Check Repair Status. 🔧

As you haven’t given any specific hardware details I’m going to offer a few basic options.

• make sure you have a usb drive backup of important files and a Time Machine backup.
• If you have any antivirus or cleanup apps installed, uninstall them according to the manufacturers instructions.
• restart in safe mode followed by a normal startup
• see if the problem is still present

Be careful of using apps like MacKeeper since many times they will interfere with the normal operation of macOS and cause more problems than they solve.

As for the cloud, you should not rely on the cloud always being there. Yes, it can be nice to be able to access files from anywhere and from any device, but the cloud is always a risk since you have no control over it as you have just discovered. Google especially has a lousy track record of dropping products & services at a whim. Also, you need to make sure you have a backup of any important files even if they are stored in the cloud, because a cloud file syncing service is not a backup especially if you cannot trust them being there when you really need it. Also, restores from the cloud are typically very slow.

Many cloud services do require files to be downloaded direct to the main boot drive and not to any other media, or they use the boot drive as a temporary download location before relocating them to the location of your choice even if you can select the external media as the destination.

Glad you were able to sort things out.

People should always have frequent and regular backups of their computer and all external media (including the cloud) which contains important & unique data.