Disk Utility Operation failed with status 5

I am attempting to back up my iMac drive with Disk Utility before reinstalling an apparently corrupted OS, but get "Operation failed with status 5." I am attempting to back up onto a new 8TB external hard drive, so I cannot believe space is the problem. Before trying to create the backup (new image), I ran First Aid on my internal drive with no problems reported. Per instructions I found elsewhere, I told the utility to create a compressed (new) image of the drive. onto the external drive. Please help. Thank you