The limit is mostly disk speed. Since you're reading and writing to the same disk, the speed is reduced. The time might approach double-digit hours. Definitely not NTFS. I already noted that the "easy" ntfs driver is read-only. There's a better one available, but even the "good" one is still incomplete -- and intentionally difficult to get. They had to do that for their own sanity. People were ignoring an amazing quantity of red flashing "USE AT YOUR OWN RISK" warnings and getting all mad when important data was lost. All it does is load the ability to understand XFS filesystems, if available. It's safe. All dd does is this:It dumps the contents of sda into sdb with complete ignorance of the meaning of data in sda and complete disregard for any current contents of sdb. (getting them backwards would be very bad. Hence the chmod step to prevent disaster.)

The way disks and partitions work means it doesn't have to understand what's in sda is to make a perfect copy. Dumping it raw replicates everything with perfection, including boot sectors, boot loaders, partition layouts, partition types, hidden recovery partitions, empty space, remnants of deleted files sitting in empty space, etc, including any things you didn't know about or didn't care about. The only caveat is the destination disk must be of equal or larger size than the source.

This single-minded blindness means it can even clone a system alien to it. Contents that seem like nothing but garbage will be replicated faithfully. I regularly do replications of Windows machines from Linux disks when installing a larger hard drive -- clone the old disk onto the new one and enlarge the partition later. I've done replications with macs once or twice.

Yeah. An up-to-date clone of the server would be a nice thing to have handy if you're trying to set up a RAID. If the RAID controller does anything unexpected like blank your drives when you configure an array, you can dub your backup back onto it to get everything back. It'd hardly be a RAID if you didn't mirror or strip or extend the disks in some way. It'd just be an extra hard drive.

If you want to keep it simple, a hardware RAID mirror would be simple. It'd act like the system you already have, a one-disk system, and all the advice I've given you so far would still apply to it. There's more complicated, fault-tolerant setups like raid5, but this takes a lot more disks and hardware, money that's probably better spent on things like a good uninterruptible power supply and a fire safe for your backups. Absolutely. A hardware mirror can swallow a single-disk failure and keep going, plus weekly backups to your external drive can save you from more drastic things.