Do u mean “so it cannot be booted externally unless u supply the password”? So erase and encrypt an HD means that the contents of the HD can not be accessed unless a password is supplied and is a general method.
Yes, this is what I am after.
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I have a new Mac Mojave, internal APFS SSD with 500GB data and a new external HD formatted HFS+ for use as time Machine backup. I expect the initial backup will take days.
Q1) I understand that APFS uses pointer to mark off the initial volume to be backuped, subsequent changes to that volume are stored separately. So during the time that time Machine is doing its backup, can I use the Mac to do other works or it is prohibited?
Q2) if I remove the external drive and go to work with the Mac, when I come home, can I reconnect the external drive to the Mac and resume the time machine backup or I have to start from scratch, I.e. a time machine backup can not be interrupted ?
The method of encrypting the drive that is suggested in the CCC documentation enables File Vault. File Vault and encrypting the drive during the erase process both encrypt the drive. There are differences between them but I don't think the encryption itself is any different.
I don't think initial backup of 500GB in Time Machine will take days. If you try to encrypt a Time Machine backup that was not encrypted - I think people report that Time Machine tells them it will take a very long time.
You can use the computer while doing a Time Machine backup. To my knowledge, Apple hasn't documented exactly how Time Machine uses APFS snapshots when doing a regular backup, but at least one reliable source says that if the disk being backed up is APFS, it will make a snapshot, compare that to a previous snapshot and then copy the new/modified files. But the ability to use the computer while Time Machine is doing a backup is not new to APFS.
If you're doing an incremental backup, it's best to wait until the backup is done before removing the backup disk. Incremental backups usually don't take a long time unless you're doing the backup after an OS update, if Time Machine is pruning old backups or unless you've put a lot of data on the disk. In any of these cases, you should schedule a manual backup appropriately so that the next incremental backup won't take as long. If you find that backups are occurring when you want to take the computer away, you should consider re-scheduling the backups or just doing manual backups when it is convenient. On the Internet, at least some people say stopping the backup mid-way through a backup forced them to restart the backup from the start. Because of this, I assume that it's possible that a restart of the backup may be required when it is interrupted.
With CCC and a backup APFS disk, you just do a normal cloning operation and have snapshots enabled on that disk. But I think you have to properly schedule how the snapshots are done and that would have to consider how you often you do backups. This can be pretty complicated (there are several different settings) and I just started using APFS snapshots on the backup disk in CCC. When I get more experience in using it, I might write something up on this. If you are going to use CCC on a backup SSD with APFS, I can look into what would be good settings for you but I would need to know exactly how much data you have, how big your backup SSD is (2TB I saw earlier) and how dynamic your data is - is large amounts of your data going to be changing often. Also, how often do you want to do a backup? But it looks like you haven't made a decision yet as to whether you're going to use Time Machine or CCC for your 2TB SSD.
About your question on SIP. Do you actually need to copy a previous Time Machine to a new disk? You might just want to keep your old Time Machine disk and start a new one. Because of the problems that I encountered with copying Time Machine backups that I experienced in Mojave, for you, I think you would be better off just starting a new Time Machine disk.