Can I use 2 harddisks to backup using time machine (not duplicate, but to extend) I have 2 x 500 GB external Harddrives and 500 GB internal harddrive . I use time machine to backup from the internal 500GB and one external 500GB HD to the second 500 GB HD. This is good till date since I only had to back up less than 250 Gigs in total. 

What would happen if and when I exceed the disk space of 500 Gigs? 

Can I use another 500 GB harddisk (in order to horizontally scale) on Time Machine . I need incremental backups like TimeMachine does and I like the interface too. I picked out 500 Gigs as an example. 


Can I mount 2 external HDs as a single one so that the TimeMachine
does not need to know about it ?
Does TimeMachine support backing up
to use multiple harddisks depending on space constraint ?
How is this problem usually addressed for a non-business requirement ?


Sairam
Yes - there are two ways to do this.

Apple intends and designed Time Machine to work with disks in a sequential manner. You are expected to fill up the first disk and put it on the shelf. Then attach the second and fill it up. You can then go back to the first and this lets you have more history before the oldest backup is reclaimed due to space running out.

An alternative is to use Mac OS X built in RAID functionality. You will have to erase all the disks when you put them into a RAID the first time - but you can have two disks with half the data written to each every time a file gets written. This is striping or RAID 0.

Using a RAID would mean both drives need to be connected all the time. You would then have one large backup and since Time Machine won't be saving two copies of many files - you will get much longer time for storage using a RAID than the first method of using the drives sequentially. If your boot drive is almost full - I've seen times where the first method could store a month or two of history, but years would fit on the second method. It really depends what files you have and how often they change.