Backing up an external HD that's not always connected?

Question

I just bought a Time Capsule to backup my MacBook and an external HD. I've set up Time Machine to backup the MacBook and external HD. But the external HD isn't always connected to my MacBook. When Time Machine runs its hourly backups, if at that moment the external HD is not connected, it backs up just the contents of my MacBook.
I've just started this process, rather than learn by trial and error I'm hoping some of you have already done something similar and can share your experience. I have a couple of concerns:
1) If Time Machine backs up my MacBook (MB) and external hard drive (EHD) and then, on a second run, with the EHD disconnected, backs up only the MB, the next time a backup runs while the EHD is connected, will it have to start all over again copying all of the contents of the EHD? Or can it recognize the EHD content which is unchanged from the last time it did a backup with the EHD connected?
2) When Time Machine runs out of space and starts deleting old backups, is there a risk that this way I could lose a backup copy of the EHD? For example suppose Time Machine backs up my MB and EHD. Then say for a couple of weeks I don't connect the EHD and meanwhile TM keeps doing backups and runs out of space. When it throws out the oldest backups, if none of the more recent ones contain data from my EHD, do I thereby lose that backup data?
Apologies in advance if this question has already been answered elsewhere. I've tried searching for answers but all my searches on "time machine external hard drive" just turn up results about using TM to back up to an external HD or else talk about what format a drive needs to be in order to get backed up via TM.
p.s. I use the EHD to store, among other things, my iTunes library. So in practice I rarely go more than a day of using the MacBook without turning on the EHD. Still, I'd like to understand how TM manages backing up of drives that aren't always available.
Thanks,
Ben Ellis

Answer 1

ben_ellis wrote:
I just bought a Time Capsule to backup my MacBook and an external HD. I've set up Time Machine to backup the MacBook and external HD. But the external HD isn't always connected to my MacBook. When Time Machine runs its hourly backups, if at that moment the external HD is not connected, it backs up just the contents of my MacBook.
I've just started this process, rather than learn by trial and error I'm hoping some of you have already done something similar and can share your experience. I have a couple of concerns:
1) If Time Machine backs up my MacBook (MB) and external hard drive (EHD) and then, on a second run, with the EHD disconnected, backs up only the MB, the next time a backup runs while the EHD is connected, will it have to start all over again copying all of the contents of the EHD? Or can it recognize the EHD content which is unchanged from the last time it did a backup with the EHD connected?
it's supposed to be smart and recognize the old backups and not redo the entire backup of the EHD in such situation. but sometimes TM does not do that for whatever reasons (=bugs). hopefully it won't affect you.
2) When Time Machine runs out of space and starts deleting old backups, is there a risk that this way I could lose a backup copy of the EHD? For example suppose Time Machine backs up my MB and EHD. Then say for a couple of weeks I don't connect the EHD and meanwhile TM keeps doing backups and runs out of space. When it throws out the oldest backups, if none of the more recent ones contain data from my EHD, do I thereby lose that backup data?
yes, in that scenario you will loose all backups of the EHD. that's why you need to make sure that your TM drive is big enough and has plenty of space to store incremental backups so that this is unlikely to happen. and back up the external regularly.
Apologies in advance if this question has already been answered elsewhere. I've tried searching for answers but all my searches on "time machine external hard drive" just turn up results about using TM to back up to an external HD or else talk about what format a drive needs to be in order to get backed up via TM.
p.s. I use the EHD to store, among other things, my iTunes library. So in practice I rarely go more than a day of using the MacBook without turning on the EHD.
Still, I'd like to understand how TM manages backing up of drives that aren't always available.
Thanks,
Ben Ellis