Can't do back up, says I need more space but i excluded everything

Question

Hi there, I recently had some issues with my iMac. It had a 500GB HD and I was using external 1TB for my backups. I had to do an Archive and install so I had the shop put in a 2TB and transfer over my stuff. So alls good. I had just under 500 GB and I was still able to use my time machine as a backup drive. But I later transferred over a load of movies and files that brings my drive to about 1.5 TB. However in time machine pref I excluded most of these files so it is well under 1 TB for sure if not around the 500 - 600 GB mark. However when I try to o a back up it says "This backup is too large for the backup disk. The backup requires 1.57 TB but only 534.56 GB are available" But I exlcuded everything big, when I go to options it tallys up saying in order to do a full back 306.9 GB is required. Any thoughts how to make this thing work cause I hate to not have backups and I don't feel like buying a 2TB external drive. Thanks for any help in advance.
Nat

Answer 1

First, never use your TM backup drive to store other files. If you must use a single drive for both storage and backup, then partition the drive with one of those partitions devoted exclusively to TM. That volume should be at least twice the size of your boot volume.
About TM "Backup Drive is Full" Alert
TM only deletes older files if they have been deleted from the source and when TM 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.
So, how long a backup file remains depends on how long it was on your Mac before being deleted, 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.
Note, that on a Time Capsule the sparsebundle grows in size as needed, but doesn't shrink. Thus, from the user's view of the TC it appears that no space has been freed, although there may be space in the sparsebundle.
Once TM has found 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 your backups anew or replace the drive with a larger drive.