not sure about if you had to format. it will work with exporting the database and account activator as I've tried it. I don't know about just dropping the saves in their place though. afaik, trophies are lost. however, there's a local trophy database that may be retained, but unlike saves, it will not survive a database rebuild.
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I don't understand fully how trophies work, but what's unlocked and everything should be in the db. the trp files are encrypted though, so I don't know how it goes from that to being placed in your database. the vita works the same way. you can manipulate trophies, even unlock them with the db, but what's actually been unlocked or how it can tell when you rebuild the database, that I don't know for either system. the database is what's being read when you look at your trophies, but rebuilding the database fixes db errors in all databases, so there's obviously some sort of record for it to go by, which is most likely related to pfs I'm sure. rebuilding the database also fixes pfs errors.
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both
@KiiWii and I have tutorials on account activation and save mounter. I reformatted my hdd on the pc to get rid of a banned account, then initialized it with firmware reinstall. after installing all of my games (including updates, this is important, because
@Leeful found that my trials save wouldn't work without the game update), use account activator and save mounter. I tested every save, and all of them work. however, I've heard of some rare occurrences where a save wouldn't work. afaik, no one knows why that particular save doesn't work, but it should work for almost all saves. chances are you won't encounter one of those.
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@triften Any backups you make will be useless if you reformat your system unless you restore your actual profile too.
Any type of backup you make is tied to your current unique internal profile information. It will restore perfectly to the system as it is now but if you format/initialise your PS4 your internal profile info will be different so any backup you make will be useless.
As
@godreborn mentioned earlier, the best way to backup your saves is via USB with an activated account and to backup your profile using the official backup/restore method.
That way if you ever initialise your PS4 in the future you can restore the backup of your profile and then restore your saves from USB or any other backups like trophys etc.
is there a way to change account id for trophies? I think my trophies are gone though, cuz I made a new backup with the new account, but I'm curious. it's not a big deal if they are, since I have maybe 50 hours of total playtime.