Time Capsule and other network storage appliances are becoming very popular for providing shared "personal cloud" storage. Naturally, this storage looks very appealing as a backup destination. The thought of backing up all of your stuff without having to plug in a cable is very alluring. Indeed, this storage is well suited for the sharing of media files, but there are some logistical and practical hurdles to backing up large amounts of data, as well as backing up macOS system data to these devices. There are also some logistical hurdles to restoring a full system backup from a network volume. Below we explain how to back up macOS system files to a network volume, then we describe some of the limitations and performance expectations of this solution.
Note on bootability: You cannot boot your Macintosh from a backup kept on a network volume. If you require a bootable backup or if you ever need to restore system files, you must use an external hard drive enclosure attached directly to your Mac to create a bootable backup.
As described in this section of the documentation, CCC will only back up system files to or from locally-attached HFS+ or APFS filesystems. To meet this requirement, you can ask CCC to create a disk image on your destination network volume. A disk image is a single file that contains the contents of another hard drive. When you double-click on a disk image file, the disk image's volume is mounted locally, as if it were an external drive attached directly to your Mac. To back up to a disk image with CCC:
- Choose your source volume from the Source selector.
- Select New disk image... from the Destination selector.
- Unless you're making an archival backup of your data, choose the option to create a read/write sparse bundle disk image file. If that option is unavailable, choose the option to create a sparseimage file instead.
- Specify the location where you want to save the disk image file. You will find your network volume in the sidebar of the New disk image save dialog*.
- When you click the Clone button, CCC will create a disk image on the backup volume, back up the specified data, then unmount the disk image when the task is complete.
* Instructions for gaining access to network volumes is available in the macOS Help Center. If your network volume does not appear in CCC's Source or Destination menu, consult the documentation that came with the storage device you are trying to access, or choose "Help Center" from the Finder's Help menu ("Mac Help" on Yosemite and above) and search for "connecting to servers".
"Convenient" and "fast" often go hand-in-hand, but not when it comes to backing up to a network volume. There are several factors that can greatly reduce the performance of your backup, and this backup strategy involves several of them.
Backing up data over a wireless connection will be considerably slower than backing up over an ethernet connection. 802.11n networks support approximately 300 Mb/s of rated (theoretical) bandwidth under the best conditions, but they usually operate at much lower speeds (130 Mbps and below, which is comparable to 16 MB/s). Bandwidth drops considerably as you get further from the base station (a wooden door between your Mac and the router will cut the signal in half), and the file sharing protocol overhead will reduce your achievable bandwidth yet more. So practically speaking, you're lucky to get 8 MB/s over a wireless connection while sitting right next to the base station. If you're running Yosemite or later, that performance could be cut in half due to Apple Wireless Direct Link (AWDL), which causes the Airport card's interface bandwidth to be shared between your ordinary WiFi network and an ad hoc network hosted by your Mac.
We performed a simple bandwidth test to a fourth generation Airport Extreme Base Station (802.11n) to demonstrate the performance decline. We copied a 100MB file to an external hard drive attached to the base station via USB in three scenarios: 1. An ethernet connection to the base station, 2. Sitting a few feet from the base station, and 3. Sitting across the house from the base station (~35 feet, no line of sight to the base station). The results were 6.5s (15.5 MB/s), 18.7s (5.3 MB/s), and 256s (0.39 MB/s) for the three scenarios, respectively. So, before you try to back up over a wireless network, consider running a simple test in the Finder to see just how fast your connection is. If it takes more than a minute to copy a 100MB file, your connection is too slow to be practical for backup purposes.
Backing up to a disk image on a network volume is often faster than backing up to a folder on the network volume. We explain this performance difference in Slow performance of network appliances can be mitigated by backing up to a disk image section of the documentation. The instructions provided above explain how to back up to a disk image, so as long as you follow those directions, you'll get the best performance possible.
The performance of the initial backup can be improved by attaching the storage directly to your Mac. If you're planning to attach an external disk to your wireless network router (e.g. base station, Time Capsule, etc.), consider attaching that storage directly to your Mac for the initial backup. Be sure to back up to a disk image on this storage (as described in the instructions above). After the initial backup has finished, you can then attach the external hard drive to your base station and specify the disk image as your destination in CCC by choosing Choose disk image... from CCC's Destination menu.
Some NAS devices cannot accommodate large sparseimage disk images. Sparseimage file are large, monolithic files, and some NAS devices may impose a 4GB or 2TB file size limitation, which will limit the amount of data that you can back up to a disk image. We recommend using a sparsebundle disk image when backing up to a NAS volume to avoid these limitations.
There are some logistical challenges to restoring a full system from a disk image backup. In particular, you must have some other installation of macOS that you can boot from in order to perform the restore — you can NOT boot from your disk image backup on the Time Capsule or other network device. Before you have to rely on your backup in a catastrophe, prepare yourself for this logistical challenge. Learn more about this in the Restoring from a disk image section of the documentation.