Why does a copy operation to a directory that serves as a mountpoint not copy the data to the mounted drive?

Question

Why does a copy operation to a directory that serves as a mountpoint not copy the data to the mounted drive?

I bought a 2 terabyte drive and mounted it in a subdirectory within my home directory.

Where /dev/sdb is my 500GB system drive and /dev/sda is my 2TB data drive:

Partition        Mountpoint
/dev/sdb1   ->   /
/dev/sdb3   ->   /home
/dev/sdb2   ->   swap
/dev/sda1   ->   /home/data

This all seems to work, and even shows up in df -h properly (i.e., /dev/sda1 is mounted on /home/data [to regenerate the fstab I booted into my arch disk live environment and mounted the partitions to the folders in /mnt that I wanted to partition, running genfstab -U /mnt > /mnt/etc/fstab; it worked])

Last night I set my box to running a 650GB copy operation to /home/data. Imagine my surprise when tons of copy operations failed due to being out of diskspace.

df -h shows that /dev/sdb3 is full but /dev/sda1 is nearly empty (77MB). The mount point is functioning properly, so far as I can tell, but the copy operation put all the data in /dev/sdb3! Presumably, if I unmount the drive, the music will still be in /home/data.

Clearly there is something about mounting and fstab that I am not fully understanding.

The particular entry in fstab reads:

# /dev/sdb1
UUID=<UUID>    /            ext4     rw,relatime 01

# /dev/sdb3
UUID=<UUID>    /home        ext4     rw,relatime 02

# /dev/sda1
UUID=<UUID>    /home/data   ext4     rw,relatime 02

Before regenerating the fstab, I had a swap entry in fstab. I'm not sure why it didn't regenerate.

Update: I managed to get the output of mount:

proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
sys on /sys type sysfs (rw,nosuid,nodev,noexec,relatime)
dev on /dev type devtmpfs (rw,nosuid,relatime,size=4051032k,nr_inodes=1012758,mode=755)
run on /run type tmpfs (rw,nosuid,nodev,relatime,mode=755)
/dev/sdb1 on / type ext4 (rw,relatime)
securityfs on /sys/kernel/security type securityfs (rw,nosuid,nodev,noexec,relatime)
tmpfs on /dev/shm type tmpfs (rw,nosuid,nodev)
devpts on /dev/pts type devpts (rw,nosuid,noexec,relatime,gid=5,mode=620,ptmxmode=000)
tmpfs on /sys/fs/cgroup type tmpfs (ro,nosuid,nodev,noexec,mode=755)
cgroup2 on /sys/fs/cgroup/unified type cgroup2 (rw,nosuid,nodev,noexec,relatime,nsdelegate)
cgroup on /sys/fs/cgroup/systemd type cgroup (rw,nosuid,nodev,noexec,relatime,xattr,name=systemd)
pstore on /sys/fs/pstore type pstore (rw,nosuid,nodev,noexec,relatime)
bpf on /sys/fs/bpf type bpf (rw,nosuid,nodev,noexec,relatime,mode=700)
cgroup on /sys/fs/cgroup/blkio type cgroup (rw,nosuid,nodev,noexec,relatime,blkio)
cgroup on /sys/fs/cgroup/devices type cgroup (rw,nosuid,nodev,noexec,relatime,devices)
cgroup on /sys/fs/cgroup/net_cls,net_prio type cgroup (rw,nosuid,nodev,noexec,relatime,net_cls,net_prio)
cgroup on /sys/fs/cgroup/pids type cgroup (rw,nosuid,nodev,noexec,relatime,pids)
cgroup on /sys/fs/cgroup/hugetlb type cgroup (rw,nosuid,nodev,noexec,relatime,hugetlb)
cgroup on /sys/fs/cgroup/cpuset type cgroup (rw,nosuid,nodev,noexec,relatime,cpuset)
cgroup on /sys/fs/cgroup/memory type cgroup (rw,nosuid,nodev,noexec,relatime,memory)
cgroup on /sys/fs/cgroup/cpu,cpuacct type cgroup (rw,nosuid,nodev,noexec,relatime,cpu,cpuacct)
cgroup on /sys/fs/cgroup/rdma type cgroup (rw,nosuid,nodev,noexec,relatime,rdma)
cgroup on /sys/fs/cgroup/freezer type cgroup (rw,nosuid,nodev,noexec,relatime,freezer)
cgroup on /sys/fs/cgroup/perf_event type cgroup (rw,nosuid,nodev,noexec,relatime,perf_event)
systemd-1 on /proc/sys/fs/binfmt_misc type autofs (rw,relatime,fd=44,pgrp=1,timeout=0,minproto=5,maxproto=5,direct,pipe_ino=13569)
mqueue on /dev/mqueue type mqueue (rw,relatime)
debugfs on /sys/kernel/debug type debugfs (rw,relatime)
tmpfs on /tmp type tmpfs (rw,nosuid,nodev)
configfs on /sys/kernel/config type configfs (rw,relatime)
hugetlbfs on /dev/hugepages type hugetlbfs (rw,relatime,pagesize=2M)
/dev/sdb3 on /home type ext4 (rw,relatime)
/dev/sda1 on /home/data type ext4 (rw,relatime)
tmpfs on /run/user/1000 type tmpfs (rw,nosuid,nodev,relatime,size=811560k,mode=700,uid=1000,gid=1000)

Answer 1

You mention that your copy command is cp -r /mnt/music data/ when you're in ~.

This means that you're copying your data into /home/<username>/data, since ~ would expand to /home/username.

However, your external drive is mounted as /home/data, according to the mount output you supplied. To finish your goal, you need to do two things:

• copy all the data from /home/<username/data to the /home/data directory.
  • rsync -avHP /home/<username>/data/* /home/data/ (or some variation)
  • this moves all the data out of your home partition and into the external drive.
• fix up how you want to access the drive
  • leave it mounted as /home/data, and just access it that way
  • one option would be to create a symlink in ~ to access it: ln -s /home/data ~/data
  • another option would be to edit your fstab to set the mountpoint for the external drive to /home/<username>/data