So you have installed Manjaro and you realize that your second disk is not writeable. You can access the data but you cannot change it.
The primary cause is permissions and those can be solved by this guide.
However the issue could also arise when you dualboot with Windows, and the partition you want to access is the Windows root partition.
If your disk is a Windows disk - it can be as simple as
$ lsblk sda 8:0 0 223,6G 0 disk ├─sda1 8:1 0 512M 0 part ├─sda2 8:2 0 60G 0 part ├─sda3 8:3 0 130G 0 part └─sda4 8:4 0 33,1G 0 part [SWAP] sdb 8:16 0 477G 0 disk └─sdb1 8:17 0 419,1G
Create a folder for the purpose (change all references to mydisk as you would want a more descriptive name).
sudo mkdir -p /data/mydisk
Replace the yX with the device and the partition number you found.
$ sudo mount /dev/sdyX /data/mydisk
Recursive change owner on the folder structure. Manjaro uses a group with the same name as the user which you can verify with:
$ groups sys lp wheel network video optical storage scanner power $USER $ sudo chown -R $USER:$USER /data/mydisk