What Happens to Time Machine Backups When an External Drive that's supposed to get backed up (e.g. Photos Library SSD) Is Temporarily Unavailable?

[Alvi]

Hi all,

I’m using Time Machine to back up my Mac via my Synology NAS, and I have my full Photos Library stored on an external SSD. This drive isn’t always connected — sometimes I disconnect it for weeks at a time.

My question is:
If the external SSD is not connected when Time Machine runs a backup, will it delete or remove the previous backup of that drive or the Photos Library? Or does it simply skip it and wait until it’s reconnected? Will the next time I connect the SDD and do a backup, do a whole 1TB+ backup or just the changes of this drive that was unavailable until now?

Thanks in advance!

 

[wonderings]

It will not delete it, at least not immediately. If it was to be disconnected for a long time, long enough for the backup to need to make space it would delete old backups first and then go forward.

 

[FreakinEurekan]

It'll be fine for "short-term" disconnects.

If it's disconnected more than a few weeks, it may not be able to catch up with changes. I've seen Time Machine just "Processing" for days on end, if it's not backed up in several weeks, to the point that it's easier to erase the Time Machine disk & start over with the backup from scratch.