I am considering updating from Sierra to Mojave and have been looking at external hard drives to backup using Time Machine before updating. Any suggestions on best way to do this update? Been looking at Seagate Backup Slim Plus for Mac Portable External Hard Drive for backup. Another question - can the same drive be used to backup system before update and then again once update completed? Or should I use separate external drive for after update just in case of something going wrong or whatever? And what size would be recommended - 1T, 2T or 4T? Have been reading some posts here regarding going back to Sierra/High Sierra from Mojave, which makes me apprehensive about doing the update. Feedback regarding this switch would be appreciated. Thank you.
MacBook Pro 15", Retina Display, Mid-2015
The choice of backup drive depends, in part, on how you plan to perform the backup. If this is your only backup drive (meaning you're not backing up at all now) then you have a choice: are you going to rely solely on Time Machine, or will you clone the drive using software like Super Duper or Carbon Copy Cloner, or will you do both? Personally, I recommend both methods (Time Machine for convenience, clone for instant recovery if something goes wrong). General rule of thumb for Time Machine is size that is twice the size of your internal disk; a clone only needs to be as large as your internal disk. You can do both Time Machine and cloning to different partitions on the same destination disk, though this is then a single point of failure; separate disks will be unlikely to both fail at the same time.
If you buy a disk that is 3x to 4x the size of your internal disk, and then partition it so that one third is for cloning, and two thirds are for a Time Machine backup, then you can clone your Sierra install so that you have something to fall back on, and turn on Time Machine in Mojave with the larger partition. That way you have something to fall back to if necessary (the Sierra clone), but you also are backing up in Mojave going forward (using Time Machine). At some point when you feel comfortable that you'll be sticking with Mojave, start making Mojave clones on a regular basis and overwrite the old Sierra clone. I would predict that you'll be pretty happy with Mojave, however.
