It worked!
It’s more of a workaround than that it solves whatever’s wrong, but transmission works, and that’s what matters to me 
For anyone reading this who’s got the same problem:
I suggest trying all options first, but here’s what I did:
ln -s /mnt/USBHDD/Torrents /home/ocms/Torrents
This creates a file/directory called Torrents in the /home/osmc folder (where transmission can actually store stuff) that links to the folder “Torrents” on the external hard drive.
It seems that all the data is actually stored on the external hard drive (first time I downloaded anything to /home/osmc it ran out of space in no time, now it’s running steadily for a while already)
Please note that I sold my soul using chmod 777 (allows everyone to read and write on the external drive. in my case it’s only those who know the wifi password), so that might have had something to do with it.
I’ll consider the topic closed and my question solved for now
Thank you everyone!
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Then have you checked your config file for transmission?
You can change settings there.
Its in home/osmc/.config/transmission-daemon/settings.json there you should
be able to change your download directory.
Make sure the transmission-daemon is not running when you alter the file otherwise nothing is stored. You can make sure the transmission-daemon is not running by.
sudo /etc/init.d/transmission-daemon stop
But I just saw there is none in my new installation after having installed it!
So is that the problem maybe, that there is no transmission-daemon started visible in the processes?, Simply because its not started on boot up??
or is it in started in systemctl now?
When I open the settings.json file you pointed to everything seems in order, the download location I set from my browser (via IP-OF-PI:9091) matches the one in the file.
I have no idea where transmission-daemon is located now, but it starts whenever I boot osmc.
All I have to do now is find out how to edit the samba settings so that it shows the external drive that is now mounted at /mnt/USBHDD.
When it was automatically mounted to “/media/Seagate Expansion Drive” samba shared it automatically, now it doesn’t…
This is how i would do it.
sudo nano /etc/samba/smb-shares.conf
[USBHDD]
browsable = yes
read only = no
valid users = osmc
path = /mnt/USBHDD
comment = USBHDD
sudo systemctl stop samba
sudo systemctl start samba
/etc/samba/smb-shares.conf doesnt exist…
I edited /etc/samba/smb.conf
the file now looks like this:
osmc@raspi:/etc/samba$ nano smb.conf
GNU nano 2.2.6 File: smb.conf Modified
write raw = Yes
strict locking = no
min receivefile size = 16384
use sendfile = true
aio read size = 2048
aio write size = 2048
socket options = TCP_NODELAY IPTOS_LOWDELAY SO_RCVBUF=131072 SO_SNDBUF=131072
[osmc]
browsable = yes
read only = no
valid users = osmc
path = /home/osmc
comment = OSMC Home Directory
[automount template]
browseable = yes
-valid = no
valid users = osmc
path = %P
hide files = /$RECYCLE.BIN/System Volume Information/desktop.ini/thumbs.db/
[USBHDD]
browsable = yes
read only = no
valid users = osmc
path = /mnt/USBHDD
comment = USBHDD
Add custom shares in smb-shares.conf instead of editing smb.conf so they will not be
overwritten by samba updates. You can only add new shares to smb-shares.conf, not chan$
the default shares or global options. If you need full control see smb-local.conf abov$
include = /etc/samba/smb-shares.conf
EDIT: I can’t get the big text to be normal sized again, sorry 
in the end it says include = /etc/samba/smb-shares.conf, even though it doenst exist when I use cd to inspect the folder… Weird?
Anyway,
when using andsmb on my phone I can now read the hard drive just fine. (can’t copy-paste anything, but that could just be the app). Windows seems to be (of course) buggy, but I’lll probably be able to work that out.
Thanks!
EDIT 2: got windows to work too, everything does what it is supposed to!
I knew it didn’t excist by default, but if you did as i said it would been created and as you read the last line in smb.conf it is included so any share in smb-shares.conf will be asif it was in smb.conf. But wouldn’t be overwritten if there is an update that changes smb.conf.
Anyway if that works for you, go with it. Good luck