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Hi! I’m planning to upgrade my drive (750 GB HDD SATA III to a SDD. However I want to keep my files. HDD stays on Mac OS Extended (Journaled, Encrypted) format after updating to High Sierra, can I clone it to an SSD which have APFS format? And be bootable?

No! You can’t use cloning software as it won’t properly transfer the encrypted files and it also won’t setup the drive correctly. First you’ll need to setup a TimeMachine backup drive so you can backup your stuff (not encrypted!) Then setup a OS installer USB thumb drive with Sierra not High Sierra! SATA based systems like yours has issues with High Sierra’s new APFS file system stick with HFS+ You talk about also having your older HDD also being used here, how are you planning on doing this?