Time Machine no longer starts

[jayducharme]

I've never had this problem until the latest Catalina update. I want to make a Time Machine backup of a 2016 iMac in our lab. I've been doing this for years with no issues. But now, the backup won't initialize. I get an error saying that Time Machine needs 1.22 tb and the external drive is only 999 gb. There's only 152 gb of data on the iMac. And it's the same iMac and the same external drive (erased) that I've been using all along. Anyone have any idea what's going on? I've tried resetting Time Machine, and trying different drives, but I keep getting the same error.

 

[HDFan]

I've tried resetting Time Machine, and trying different drives

What is the size of the drives you tried?

 

[jayducharme]

They're all 1 tb drives, and I used them previously with no problem.

 

[HDFan]

Time Machine guesstimates how space it is going to need and compares it to what is available. Unfortunately its' estimates of space needed are often inflated. Maybe in Catalina the estimating algorithm is even more off.

There's only 152 gb of data on the iMac

where are you getting this number? Can you post what you see in Finder's Get Info?

 

[jayducharme]

Maybe in Catalina the estimating algorithm is even more off.

That’s what I’m thinking.

The size estimate indeed came from Finder. For the heck of it, I tried out Carbon Copy Cloner. Several other discussion boards recommended it. And it had no trouble making a backup (which totaled about 158 gb). It was also a LOT quicker than using Time Machine (though not as simple to use), so I might just stick with that for now.