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.
What is the capacity of your backup drive and how much space is available. The error is not about your startup volume, Macintosh HD.
The capacity of my backup drive is 148.2 GB and 148.2 GB are available. There are 0 files on the drive I am trying to back up to right now.
my startup volume has 136.14 GB used and 26.73 GB unused.
I am not using 163.3 GB of storage on my startup volume, only 136.14 GB, which is why I am confused by the error message.
I have not used the drive for Time Machine backups previously so there are no previous Time Machine backups that would get in the way.
Might I suggest you try this:
1. Open Disk Utility in your Utilities folder.
2. After DU loads select your hard drive (this is the entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the DU main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to one. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Click on the Partition button and wait until the process has completed.
4. Select the volume you just created (this is the sub-entry under the drive entry) from the left side list. Click on the Erase tab in the DU main window.
5. Set the format type to Mac OS Extended (Journaled.) Click on the Security button, check the button for Zero Data and click on OK to return to the Erase window.
6. Click on the Erase button. The format process can take up to several hours depending upon the drive size.
When done quit DU, then use the Time Machine preference panel to select the drive as the Time Machine backup device, configure any exclusions you may want in the Exclude list, then give the backup another try.