I used to think this was a given, too, but I’ve discovered that many (most) NAS use a Linux OS, and therefore typically only (on lower-cost models) allow you to format the disks in EXT[2/3/4] format.
On a NAS I mange for someone else, I found that using CCC to clone to a disk image, over time, began not only to take longer and longer and longer to complete, but also to slowly break down with cumulative errors due to timeouts reading files on the destination drive, and would then later start generating errors while writing. Upon closer examination, these files were invariably very large, and often disk images (with thousands or tens of thousands of bands), themselves.
Eventually, CCC would report the master clone disk image as corrupt, and beyond repair; attempts to repair it over the network failed; attempts to transfer the entire disk image to the local drive and repair there also failed – and all of these operations were murderously slow.
Eventually, I would just give in and delete the remote disk image (clone), and start over; a process that could literally take 3-4 days to complete (backup image around 780GB), with the deletion itself taking almost 2/3 of that time.
It finally occurred to me to check for fragmentation. Doh! Turns out, disk fragmentation was indeed the issue. But, I could only check it; I could not defrag it, since there doesn’t appear to be any defrag utilities on most of the lower end NAS available, unless you get uber geeky and install your own open source NAS software.
On a hunch, I threw in a spare SSD, and not only were the overall backups and nightly incremental backups faster (than an otherwise snappy 7200RPM drive), so were recovery and basic transfers of other data (music, movies, photos to the general household network shares).
Moreover, CCC never slowed down again, and never ran into any errors since. Now, I am concerned that there is no TRIM features for the SSD I installed, and I’d have to purchase an expensive SSD with internal TRIM features to override that worry (or purchase a new NAS with SSD TRIM support enabled), but this allowed me to keep an eight year old NAS going without spending anymore money.
tl;dr: SSDs can make any NAS that doesn’t self-defrag reliably faster and consistent in the long run since they don’t care about file fragmentation.
If you don’t want to spend money on SSDs (and it really doesn’t make a ton of sense to not put in a big, fat, 6TB-12TB drive in a device you’re using to make backups from multiple machines and possibly use as a household media/file server), just make sure that your NAS has automatic defrag capabilities, or at least the ability to manually defrag if necessary.
If anyone knows of a utility available to remotely defrag an EXT volume on a remote NAS, please let me know.