When comparing ShiftIt vs Backblaze, the Slant community recommends Backblaze for most people. In the question“What are the best power user tools for macOS?” Backblaze is ranked 10th while ShiftIt is ranked 46th. The most important reason people chose Backblaze is:
The default configuration includes everything except for system files to be backed up. That also includes external USB drives. This will work for most people. It can, of course, be customized to each person's liking.
Specs
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Pros
Pro Easy to configure
ShiftIt is configured through a simple UI where you simply press the hotkeys you wish to map to a certain action.
Pro Free
Pro Intuitive
ShiftIt is very straight-forward to use for most frequent usages.
Pro Open source
Pro Configuration can be committed to your personal dotfiles repository
Configuration winds up living in .hammerspoon/init.lua and .hammerspoon/Spoons/ShiftIt.lua. Very easy to track configuration changes.
Pro Beginner-friendly defaults
The default configuration includes everything except for system files to be backed up. That also includes external USB drives. This will work for most people. It can, of course, be customized to each person's liking.
Pro No file size or traffic restrictions
There are no limitations of file size or amount of data you can upload. The $5/mo plan gets you unlimited cloud storage.
Pro Excellent restoration performance
Backblaze creates a zip file of all your files for you to download, thus decreasing the amount of data you need to download and increasing the speed of the download since it's one connection instead of multiple that need to be opened and closed for a list of files.
Pro Secure
AES encrypted files are transferred over a secure SSL connection.
Pro Download any backed-up files from any web browser
Some backup services like Carbonite require a client to be installed to restore files. Backblaze allows you to login and download any or all files from a web browser, meaning it also serves as an ad-hoc cloud storage platform.
Pro Users can order a physical copy of their data
You can have a flash drive or an external hard drive delivered to you. They cost $99 and $189 respectively.
Cons

Con A little slow
Similar to Spectacle, but it has a few more features, such as being able to pick the size adjustment increments. Spectacle was a little faster, though. Repeated size changes or moves in succession can cause it to wait for it to complete.

Con Imprecise when readjusting sizes
Con Doesn't support moving windows between workspaces
There's no way to move a window to a different workspace with hotkeys in ShiftIt.

Con No Linux support
There's no Linux client for Backblaze.
Con Pay extra to keep deleted files permanently
Additional $2/Month plus $0.005/GB/Month for files updated, changed, or deleted more than one year ago.
Con Backblaze does not support network (NAS) drives
If you use a thing like Drobo, Backblaze will not back it up.
Con Buggy
