Cannot get Transmission find the data of my torrents

Question

I have a raspberry pi set up and I want it open 24/7 for seeding my torrents.

At first, I plugged in a 8GB NTFS usb stick and I seeded a bit from there, and everything was fine.

Then, I changed to a 320 GB ext4 external hard drive which had the torrents of the USB stick and extra ones.

The torrents that were on the USB stick as well are being recognized just fine, but when I add another torrent which matches the extra data, I get the usual

"Error: No data found! Ensure your drives are connected or use "Set Location". To re-download, remove the torrent and re-add it"

while I clearly can see that the data are there.

All the files (the ones that are being seeded normally and the ones that cannot be found by transmission) have the same user and group permissions:

Answer 1

I solved this by running:

chmod -R a+rwX /media/MountPoint

where /media/MountPoint is the mount point of my external hard drive.