To try to calm things down a bit, there are at least two types of "clone" processes. One is a block-for-block or byte-for-byte clone and the other is a file by file clone. You can definitely clone a bigger drive to a smaller drive as long as the data on the larger drive is smaller than the smaller drive in a file by file clone. That's not a byte-for-byte clone, which goes even further and requires that the two drives absolutely match in geometry (platters, heads, sectors, etc). But for a file-by-file clone, as long as it fits on the smaller drive, it works perfectly. I've done it many times before.
Thanks for the clear explanation, Jake. I guess that since I just use SuperDuper! "as is" for cloning each of my Mac's internal SSDs, along with insuring that the size of the external drive/partition is a little larger than the size of all the items on each SSD (only using about 40% of each SSD's total capacity), I was unaware of the two types of cloning processes. But I am curious as to the type of cloning SuperDuper! does. I just tried to go a google search of that, but could find nothing definitive. Could you let me know what that is?
Also, I've only once done a restore from a SuperDuper! backup, and of course it went well (no space issues at all, if that was/is a consideration). If I want to do a complete clean up of each internal SSD, I first boot the applicable Mac from the latest SuperDuper! backup I have, then Erase and Format the SSD. I will then do a clean, fresh installation of the latest version of the Mac OS I am using, and then migrate/copy all the needed files, folders, etc. from that most recent backup. Might seem like a lot of work, but given that I do exactly that when I install a new version of the Mac OS, it's second nature to me (and it works).
As for the unbootability of the internal drive, could it be that because the drive is so full that the OS is unable to establish scratch and cache space on it, hence it won't boot. If that is the case and if you get a new drive and CCC, do a full clone to a larger drive, including the system files following the directions at the CCC website, it may well boot from the external because it has the room. Then you can boot from it, then go back to the internal to look to clean up some space on the internal to make enough space for it to boot. If it fails to boot from the CCC Clone, then you will know that the internal drive has system problems that got cloned over and so will need to have a full reinstall done.
Excellent points! Regarding how "clean" one's system is, and the subsequent backup/clone, that is one of the reasons why I am doing some kind of cleanup every day on my Mac Mini (I also do it when I use my MacBook Air). So far, whenever I migrate/copy needed files, folders, etc. after doing a clean installation of the Mac OS, I've not had any issues.
It is my understanding that the op has some critical files/folders, and if they are taking up a lot of space, it would be an issue for him to remove them. He mentioned the possibility of purchasing an additional external HDD. That is one reason why I suggested above that he partition that new HDD, then clone everything on his existing external HDD onto that partition (the size of that partition should be anywhere from 10% to 20% larger). Of course, that assumes that the existing external clone is OK, as you so rightly point out. I was under the impression that he can boot from the clone.
n any event, if he can do that, then he can delete some files, folders, etc. from either the internal SSD, or if he does a clean installation of the Mac OS he is using, or from that external HDD which would be migrated back to the internal SSD.
