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(One thing worth noting: the newer APFS-format OSes cannot be backed-up on HDDs, due to how they're designed to run on SSDs. I'm not impressed with Catalina, and dissuaded from using it by all the breakage of plug-ins and such. I moved to Mojave from High Sierra mainly to go easier on my eyes with the dark mode, and everything I use survived the move except for Best Service Engine (K'd), with which I was using the Synth-Werk library, fortunately, I'd kept a partition of High Sierra on the SSD, with Synth-Werk still available to me in Logic there. I also have one of the USB ports occupied by a 256GB shorty USB drive, where my libraries go (Arturia, Gforce, Spitfire LABS, Waves, Battery, Reaktor) rather than on my system drive.
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HDDs are good for recovering lost data with Disk Drill, I have been able to recover a good 99% of data from an "erased" HDD. I myself replaced the optical drive with a 7200 RPM HDD for data. Those specs are the same as my Macbook Pro.
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