Time Machine: Does it store internally? Backup Storage Too Much

I used Time Machine to backup my data to an external Seagate HDD. The backup was successful in the view that my files are now stored to the external HDD when I checked. Upon checking my storage space (About this Mac/More Info/Storage), there was 100+ GB of space taken up by 'backups' in spite that my data was stored externally. I intended to store externally only since 256 GB total storage for my Macbook is not enough for my liking.

The good news is when I switch Time Machine off, backup does not consume any storage space. The not so good news is I do not quite understand this. Is data really stored internally or only externally (Seagate HDD)? From one viewpoint, this seems like an OS 'illusion' because when Time Machine is turned on and off, storage capacity gets consumed and disappears with the click of a finger. From another viewpoint, when Time Machine is switched on, the OS treats the computer as if in fact it lacks storage space by not allowing me to store anymore files exceeding the storage capacity.

The only reason Time Machine will write to the internal drive is when an external drive is not available and Time Machine is turned on.
If you leave the external drive connected it will offload the backups from the internal drive to the external drive.

So with you, when you use Time Machine to backup files to your external drive, I assume you turn Time Machine off?
I cannot imagine who one would leave it on as this chomps up a big chunk of storage space.

simonvee said:

So with you, when you use Time Machine to backup files to your external drive, I assume you turn Time Machine off?
I cannot imagine who one would leave it on as this chomps up a big chunk of storage space.

Click to expand...

No. Time Machine must be on in order for backups to occur. Here's what is happening:

1. Once you have set up Time Machine it backs up files to the external drive as long as Time Machine is on and the external drive is available.

2. If the external backup drive is not connected the backups are written to the internal drive. When the external drive is re-connected the backups will gradually be moved to the external drive freeing the space on the internal drive. This does not occur immediately but does happen.

3. No backups occur if Time Machine is off.

Edit: You might find this article helpful. It is for an older version of Time Machine but the basic operation is the same. That version of Time Machine did not write to the internal drive so that's not discussed.

2. If the external backup drive is not connected the backups are written to the internal drive. When the external drive is re-connected the backups will gradually be moved to the external drive freeing the space on the internal drive. This does not occur immediately but does happen.

Click to expand...

Note - this only applies to portable Macs: MacBook Pro, Air, etc. It does not apply to the iMac, Mini, or Mac Pro.

Thanks for the catch. Should have mentioned that.