I like
SynchronizeIt! from Grigsoft. Streamlined interface, with enough options for a good customization ; the main reason why I started using it years ago is that I was looking for a software that would preserve all timestamps when copying files, including folders' timestamps : SynchronizeIt! 3.5 does this by default, and is one of the very few Windows file copy tools with this behaviour. Robocopy can do it too with the /DCOPY:T switch. I use both, Robocopy is perfect for copying or updating a whole arborescence, but if there's a need to visualize things and uncheck many files, I use SynchronizeIt!. TotalCommander and WinMerge can also compare two arborescences and copy missing files, but they don't preserve all timestamps and have no such option. FastCopy does preserve all the timestamps, but the interface is stripped, it doesn't display the files which are being copied, basically it works just like Robocopy with a very simple GUI (although I don't know if it actually uses Robocopy “under the hood”) ; it also has the reputation of being faster than other GUI-based file copy tools (according to
this comparative test for instance). Most of these tools are not automated, they have to be run manually, or a planned task has to be created for them to operate at regular intervals. If you want a fully automated and constant mirroring of one drive to another, then a RAID 1 setup may be the most practical. But a real backup drive shouldn't be connected to the active computer constantly, so if you want to do this for the sake of convenience, you should still have
another drive as a backup, which will be connected once a day or once a week (depending on how often and/or how extensively you modify or add files), and otherwise be stored in another place, ideally not in your home (at work, at a friend's place, in your
nuclear shelter...), thus protecting you against viruses, electrical problems, flood, theft, divine wrath, angry ex-girlfriend...
SynchronizeIt! has one bug though, which I discovered : it systematically corrupts files with the “sparse” attribute (some downloading softwares automatically create sparse files, unbeknownst to the user), which get truncated after exactly 25000 bytes (the copied files have the same size but are filled with 00s beyond that point, and the copy speed is abnormally fast, like a 1GB video copied in a split second). In October 2015, I reported this issue to the author (actually it was the second time I reported it : first time was in 2010, and at the time I had no explanation...), who provided me with a fixed version, but for an unknown reason he hasn't released it publicly, the version available for download is still the old one, which has this bug (I asked the author recently why, but had no reply). If someone is interested, I can provide the fixed version by private message.