Time Machine couldn't complete the backup

I know this is a over-used thread, anyway here we are, by the end of 2017 and some of us (me!) still have problems...

the complete message is: "Time Machine couldn't complete the backup an error accused while copying files. The problem may be temporary. If the problem persists, use Disk Utility..." bla bla.

I am on os 10.12.6, trying to backup on a 4tb Seagate external drive, and I get the message pretty much right away after a backup starts. It doesn't even complete the first backup. I reformatted the external drive, checked it, repaired it, rebuild it, and the same with the internal drive to be backed up, still have the same problem.

I saw posts about WD drives with similar problems, but not about Seagate. On Amazon, where I bought it from, people report success with Timemachine, no one says it didn't work. I had this problem before with another computer and another backup drive, but it eventually succeeded and in the end worked. This one doesn't. I've been backup-less for over a month now.

When opening Time machine system panel, it shows some progress, and says "calculating time", but it never shows how much time it calculated, and crashes before that. I mean fails. Maybe this could be it? Also tried to back up in Safe mode, failed with the same message.

Open at any suggestions, including fixing the hard drive with a sledge hammer.

thanks

alex