Sure
Well hopefully you are learning things along the way
But the long-term goal is to make your kind of use case easy to setup I believe.
So, let’s try to mount your external drive and use it to store backup archives on it.
I think this should go like with something like this :
- Move your existing backup in some other folder (on the same storage you currently have)
- Mount your external drive to use it as
/home/yunohost.backup/archives
(the place where backup archives are stored in the context of yunohost)
- Put back you old archives (implicitly moving them to your USB stick)
First step then : you can move your existing backup archives with something like :
mv /home/yunohost.backup/archives /root/my_precious_archives
This shouldnt take too long (maybe immediate) as, for linux, it internally just changes the name of a few files.
Second step : plug your USB drive in, then run fdisk -l
.
It should show you a whole bunch of info about your drives / storage, and in particular, around the end, you should see something like Device /dev/sdb1 ... Size 30Go
(assuming this is a ~30 Go stick). This means that your USB drive is called /dev/sdb
(without the 1!) and has a partition /dev/sdb1
(and maybe some others). The name /dev/sdb
might differ on your system, but basically you should check that the sizes are consistent with the size of your USB drive, just to make sure you’re not mistaking the /dev/sdthing
with another drive !
Once you are okay with this, assuming you only have one partition on your USB drive, mount it with something like :
mkdir /home/yunohost.backup/archives
mount /dev/sdb1 /home/yunohost.backup/archives
chmod o-rxw /home/yunohost.backup/archives
Basically, the mount
line “just” tell the system to use your first partition of your USB drive to store everything that should go into /home/yunohost.backup/archives
…
Last step then ! Just do basically the reverse of the first step :
mv /root/my_precious_archives/* /home/yunohost.backup/archives/
And this will take a bit longer as it will move all your archives to your USB drive. This might take a while depending of the size of your archives. Don’t cancel this command though ! If you want to be extra careful, we can tell you a bit more about copying to the destination, then removing the source 
If you successfully do this, then the step about setting up automatic backups should be a bit easier !