Is it possible to backup Windows and Linux (both on the same HDD) without having to restart and log into each separately? If so, how can i do it?
Welcome @gummywrench
I think the technical answer is “yes” but there are implications to the various ways you could do it. If you can run it from a separate system that has networked access the storage from either/both dual booted OSes, that would allow you to have a single backup. If you have the ability to export your files from each of the dual booted OSes (via NFS, SMB, etc), that gives you a different option. If this is possible for you, stop reading and respond saying so.
Some information on which types of files you want to backup will help. (OS files, media files, network attached storage, etc)
The various options I can thin of are:
- Backup both OSes’ files only when running one of the OSes
a) You have one Duplicati instance that runs when you’re running that OS.
b) Obviously, if you rarely run that one of the OSes, you’ll have infrequent backups.
c) You would need to restore both OSes files from this OS’ Duplicati instance
d) This requires you mount the other OSes’ files in the local OS - Backup only the “local” OS from each of the OSes
a) You have two Duplicati instances, one running within each OS. You also have two different backup targets.
b) While you won’t be backing up the other OS if you stay booted on one of them for a long time, it probably doesn’t matter since that other OS isn’t changing if you don’t run it
c) You would do restores locally from each OS - Backup both OSes from both OSes
a) You have two Duplicati instances, one running within each OS. You also have two different backup targets.
b) You will have continual backups of both OSes, but this is a bit overkill given 2b above. It may give you some additional safety if one of the OSes is corrupted, you might be able to “fix” it by restoring files for it while running the other OS.
c) You can restore locally from each OS, you can restore files to the other OS from each OS
d) This requires you mount the other OSes’ files in the local OS
The best option for your own sanity is probably #2. However, if you’re sharing any files between the two OS instances, there’s some other considerations. If that is your case mention that too.
I suspect there are other possibilities here, too, with luck someone will come up with a simpler answer 
Regards,
L
mmh… restore files from one type OS (windows) to another type OS (linux, mac) is hard job: problem with path syntax, permissions, names… I supposes it’s same thing with backup process. Of course you can mount a partition and perform backup process but you can’t backup windows from windows, restart PC and backup linux from linux.
Change of Duplicati server (+ OS) without losing the remote data gets into path and permission issues. Basically a Duplicati backup keeps one type of paths. Don’t change your mind. Other things are easier.
Whether or not you care about things such as restoring NTFS ACLs and Linux permissions is your call, however if you want them without conversion risk, then I think the OS on its own filesystems does best.
Easier is if you simply want the file content. I think Linux can often read/write NTFS. Reverse is harder. There’s plenty of information on the web about this, if you want to try to set it up for Duplicati. Example:
How to Mount and Access Windows NTFS Drives in Linux
3 Ways to Access Your Linux Partitions From Windows
Are you already doing things like this? If not, then backup of an OS while it’s up might work. What else would change its data when it’s down? Of course infrequently scheduled backups complicate this plan. Scripting a backup on shutdown would solve it, but then shutdown is slower. There are many tradeoffs.
EDIT:
Topic title mentions “restart and login to both”. Why is login mentioned? Duplicati doesn’t need the login, and running it as a service would give another option (e.g. scheduled backup) if a no-login restart is OK.