In theory it should be easy. Well, relatively easy, anyway.
To start off you’ll have to make sure the drive/partition is mounted. For that you need to know the drive name and partition number of your partition. For example sda1 or /dev/nvme0n1p2.
This can be foubd out with the blkid command
blkid
The command should return a list similar to this:
/dev/nvme0n1p3: UUID="D563-DAB7" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="0653a05b-0f22-f64c-a283-7cc0dcb83551"
/dev/sda1: LABEL="5TB" UUID="953836d8-e355-4c6d-ac1a-0914b8414f50" BLOCK_SIZE="4096" TYPE="ext4" PARTUUID="7a05b57a-5368-c647-baa3-410f462004a6"
(Mine is a lot longer, I just grabbed an excerpt as an example. Also note that mine is alreeady mounted at /mnt/5TB)
From that I can discover that my 5TB s partition name and number is **sda1**. I can also see that it's of filesystem type ext4`. That is very important.
Armed with this knowledge, when in the Live CD environment, you can create a mount point for it. For example:
mkdir ~/5TB
(The ~ character is a Linux alias for your home directory. So that command would create a directory 5TB in your home directory.
If all’s well up to here, you can go ahead and mount the drive. According to the information gathered above, the command would be:
mount -t ext4 /dev/sda1 ~/5TB
This command will mount filesystem type (-t) of ext4 on partition /dev/sda1 on mount point ~/5TB.
You can them browse the files as normal on ~/5TB.
See [root tip] Use systemd to mount ANY device for more information.
Edit:
Remember that this isn’t permanent. Any changes made to Manjaro itself will not survive a reboot and you’ll have to do it over again on the next startup.