Time machine is filling up hard drive

Time machine in Mojave doesn't work the same as it was used to do Sierra version. Since High SIera came things have started to change. Apple made a change to this in High Sierra. Instead of storing snapshots on HFS+ volumes, Apple says that High Sierra only uses APFS-formatted, “all-flash” storage (i.e., not a Fusion Drive), whether it’s an internal or external flash drive.

In you situation if both the drives get filled up suddenly in one or two backups, then it must be because of the local snapshots which are stored in Mac itself and when the relevant hdd (with time machine turned on) gets connected all the snapshots are transferred to it as backup files. Remember Snapshots are only made by time machine when the Hdd is not connected and these are temporary. temporary files get deleted automatically when you reboot Mac but sometimes it doesn't and ends up using a lot of storage. Use this method to delete these :-

• Terminal (from Applications > Utilities), and then copy and paste this command and press Return:

1. tmutil listlocalsnapshots /
2. The Terminal will show a list of local snapshots with names like com.apple.TimeMachine.2018-03-01-002010. To remove a snapshot:
3. Copy and paste this command: sudo tmutil deletelocalsnapshots
4. Type a space and then paste in the date portion of the snapshot.
5. Press Return, and finally press Return.

Altogether, using the example snapshot above, that looks like:

sudo tmutil deletelocalsnapshots 2018-03-01-002010

If it deletes properly, you’ll see Deleted local snapshot '2018-03-01-002010' in the Terminal as the response.

Also You can delete backup in external hdd by this method:-

Open Terminal

1. sudo tmutil delete /Volumes/drive_name/Backups.backupdb/old_mac_name

OR delete backups one by one if you wish to delete only some of them See below:-

1. sudo tmutil delete /Volumes/drive_name/Backups.backupdb/mac_name/YYYY-MM-DD-hhmmss

Good Luck man!! :)

Time Machine deletes older files if they have been deleted from the source when it needs space on the backup drive for a new incremental backup. Time Machine "thins" it's backups; hourly backups over 24 hours old, except the first of the day; those "daily" backups over 30 days old, except the first of the week. The weeklies are kept as long as there's room.

How long a backup file remains depends on how long it was on your Mac before you deleted it, assuming you do at least one backup per day. If it was there for at least 24 hours, it will be kept for at least a month. If it was there for at least a week, it will be kept as long as there's room. By default, Time Machine backs up hourly. That cannot be changed in Time Machine. There are third-party utilities that will modify the backup interval such as Time Machine Editor.

The Time Capsule sparse bundle grows in size as needed, but doesn't shrink. Thus, from the user's viewpoint of the Time Capsule, it appears that no space has been freed, although there may be space in the sparse bundle.

Once Time Machine finds it cannot free up enough space for a new backup it reports the disk is full. You can either erase the backup drive and start over or get a larger drive.

Just a few minor corrections. High Sierra can only read an APFS formatted drive. Mojave and Catalina can both read and write an APFS formatted drive.

Snapshots will only exist if Time Machine is left active but the backup drive is not available. Snapshots are supposed to be removed if and when more storage space is required. According to Apple's information there is no need to turn off snapshots. It isn't necessary to turn off snapshots. Rather, configure Time Machine to exclude them from the backup (assuming they aren't excluded, automatically.)

Your conclusion of what is wrong is incomplete. It may also be due to the drive simply filling up and finally running out of space.

Feeling very sorry that all these didn't work. An apple expert can help or anyone with very high knowledge. I don't have much experience with CatalinaOS. But moslty this get solved by the mothod i had mentioned in all apple versions. Don't know what's going wrong with catalina as of now.

Thank you for adding these points to my knowledge. It certainly is helpful

Welcome man!!! Always happy to help others!!!👍🏻