I have a large external hard drive that I am using as a time machine backup volume. Since I don't need all the storage space for backups, I'm considering storing additional data on the drive. Is there any danger of this usage interfering with the backup system or is this an always-safe storage spot?
Seeing that external storage is relatively inexpensive, I would leave the existing TM drive alone and purchase another external drive. That way, you can backup the additional data on the new drive to the TM disk, too. You wouldn't be able to do this if the additional data resides on the TM drive itself.
If you do choose to use the TM disk for non-TM data, I recommend partitioning the drive to separate the TM data from the non-TM data. If you ever need to reformat the TM partition, you won't need to move that additional data to another disk first, or visa versa.
All that being said, it is technically possible to use the TM disk to store additional data. I used to do it until #4 (below) happened and I lost ~10k .mp3 files I then needed to re-rip from CDs. (I did re-rip about 80% of the lost files to Apple Lossless format, so it wasn't a complete disaster, but I did lose thousands of files.)
There are several drawbacks you should keep in mind:
- Time Machine will delete older backups more frequently when available space is low.
- Performance on the drive will suffer when TM is running and you are accessing the additional data at the same time.
- If you need to transport the additional data off-site, you'll also be taking your TM backup with you.
- When (not if) the entire disk crashes hard, you will lose the additional data unless it is backed up somewhere.
Yes - you are free to use that disk to mix storage and Time Machine backups. Stay out of the Time Machine folder/disk image and you will be fine.
Since Time Machine can back up to a network store and a locally connected drive - the details on where Time Machine squirrels away it's backup data is slightly different, but you are free to use the rest of either disk to store whatever files you wish.
The only down side to storing files on the same volume that is being used as a Time Machine destination is that you cannot use Time Machine to both back up those other files and store backups from elsehere. Even with the dual destination rotation scheme added in Mountain Lion - Time Machine is hands off for any drive listed as a destination.
None, as far as you know what you are doing and don’t change the name of the volume. Also make sure that the partition table type is GUID.
The only possible side effect would be your transfer speeds getting slow because a backup is happening.