I did have:

1 TB internal drive with about 250GB total of material
1 320 GB drive with Time machine backup
1 500 GB drive with CCC backup

Wanting to retain High Sierra for 32 bit applications and newer systems for future use, I reconfigured the 1TB internal drive (350BG for 32 Bit, 650GB for 64 Bit) and restored the 250GB of material to each with my CCC backup.

I configured a new external 1TB drive (350BG, 650GB) and created new CCC clones.

Everything has been checked as bootable.

For my Time Machine backups, I wanted to have the 320GB drive for one and the 500GB drive for the other. Note: My previous 350 GB Time Machine always worked, except once in a while it would say "not enough room" and I'd redo it. That's not an issue for me as I don't need Time Machine to go back forever.

But, now I have two drives that Time refuses to use for the planned TM backups. It says there is not enough room to create a TM backup. Grrrr. It seems that TM now wants to have more space than needed for the actual material.

How do I get around that issue?

Two thoughts: first, is each Time Machine drive trying to back up the full 1 TB internal drive and not just the one partition?

Second, are the TM drives empty or do they contain the previous backups? Since the partitions are "new drives", TM should want to back up everything fresh and for the first time, requiring more space. Otherwise…?

NOTE: I misspoke with the smaller drive size....it's 320GB and I've corrected the post. This is the drive that has always contained my Time Machine backup.


No.


No. I used Disk Utility to reformat but Time Machine won't put 250GB of data onto a 320GB drive..... even though it always did so before. It now says: "Time Machine backup needs 505.21 GB but only 319.30 GB are available."

From experience, Catalina is even picker about the size of the Time Machine drive and I have no idea what algorithm is used to determine the minimum drive size. Some suggestions...

1. You might try disabling the backup of system files and exclude any other attached drives from the backup set essentially limiting Time Machine to backing up ONLY your data files ie. your user folder. Your system files and applications — particularly those purchased thorough the App Store — can easily be reinstalled from the internet. That will significantly reduce the size of the backup and MAY get you below Time Machine's size requirement.
2. Move Desktop, Documents, Music library, photos library, [/I]etc.[/I] to your iCloud Drive (System Preferences > iCloud > Cloud Drive), here again this can dramatically reduce the size of the TimeMachine backup data set.
3. Get a higher capacity Time Machine drive
4. Forego versioned backups and use Carbon Copy Cloner only

It appears to me that Apple has made decisions that don't allow us to use Time Machine according to the original Mac philosophy - Users should have a computer that let's them do things the way they wish.

I have absolutely no requirement to use Time Machine as a long range history. I just want to use a drive that is more than large enough to contain more recent files and, should the drive run out of space, Time Machine can boot the oldest files off the bus.

However, I found a while ago that Time Machine wasn't dropping oldest files. Even though there would be 100GB of available space, it would say Backup Failed. But at least it allowed me to record a new Time Machine copy on the same drive that failed. Now it won't even do that.

It appears to me that Apple has decided that it knows more than I do about my backup requirements and, in order to conform, I must spend money on new drives if I want Time Machine. I'll save the dough and, as you suggested, forego versioned backups and just have an extra CCC copy.

All is not lost! If you have at least version 5.1 of Carbon Copy Cloner, you can do versioned backups. Here's what CCC says:

"New in CCC 5.1 Versioned backups with APFS Snapshots

CCC now offers support for point-in-time restores by leveraging the snapshot feature of Apple's new APFS filesystem. CCC is also the first comprehensive snapshot management utility for macOS."

There is more info in the CCC Help, but one example is versioned backups every hour, just like Time Machine. I don't know the storage requirements, but you have CCC, so it is worth looking at.

Snapshots only "save" data sectors where changes are made by changing the pointers in the file to point to the new sector and temporarily not releasing the original data sector to be overwritten, so it is very difficult to assess how much disk space they consume and even then it could be a wild variation. Snapshots are only supported on drives formatted APFS and IIRC the maximum number of snapshots you can have is 10.

I see a lot of potential in snapshots but AFIK other than command line apps, CCC is the only app to offer any access or control. Basically they are a powerful but unexplored and unexploited tool. I suspect will see Time Machine taking advantage of snapshots, but first Time Machine will have to be made compatible with APFS. One step at a time.

I'm CCC 5.1.9 but I'm not APFS and, if I don't have versioned backups, it's not a showstopper. Thanks for the thought.