Does the user under which Transmission is running have proper access right to that folder?
What does it say currently for "download-dir" in your settings.json file? The settings.json file should be somewhere in /usr/local/etc/transmission/
if you haven't changed your config dir.
"alt-speed-down": 50,
"alt-speed-enabled": false,
"alt-speed-time-begin": 540,
"alt-speed-time-day": 127,
"alt-speed-time-enabled": false,
"alt-speed-time-end": 1020,
"alt-speed-up": 50,
"bind-address-ipv4": "0.0.0.0",
"bind-address-ipv6": "::",
"blocklist-enabled": false,
"blocklist-url": "
http://www.example.com/blocklist",
"cache-size-mb": 4,
"dht-enabled": true,
"download-dir": "/mnt/krigsvolds/Media/Downloads",
"download-queue-enabled": true,
"download-queue-size": 5
"encryption": 1,
"idle-seeding-limit": 30,
"idle-seeding-limit-enabled": false,
"incomplete-dir": "//Downloads",
"incomplete-dir-enabled": false,
"lpd-enabled": false,
"message-level": 2,
"peer-congestion-algorithm": "",
"peer-id-ttl-hours": 6,
"peer-limit-global": 200,
"peer-limit-per-torrent": 50,
"peer-port": 51413,
"peer-port-random-high": 65535,
"peer-port-random-low": 49152,
"peer-port-random-on-start": false,
"peer-socket-tos": "default",
"pex-enabled": true,
"pidfile": "/var/run/transmission/daemon.pid",
"port-forwarding-enabled": true,
"preallocation": 1,
"prefetch-enabled": true,
"queue-stalled-enabled": true,
"queue-stalled-minutes": 30,
"ratio-limit": 0,
"ratio-limit-enabled": true,
"rename-partial-files": true,
"rpc-authentication-required": false,
"rpc-bind-address": "0.0.0.0",
"rpc-enabled": true,
"rpc-host-whitelist": "",
"rpc-host-whitelist-enabled": true,
"rpc-password": "{e8e8e9317a398ba42ed3f253f5079768363a57c7H1ZA4PYu",
"rpc-port": 9091,
"rpc-url": "/transmission/",