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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)
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    Are you sure the quoted fstab is correct? It also mounts sdb3 under /home/data although that is your /home partition. Commented Dec 31, 2018 at 12:45
  • Whoops. I copied it from the screen rather than from the computer. Commented Dec 31, 2018 at 13:06
  • Unfortunately, I'm at work now, so if this is something that requires a lot more information and isn't simply a known quirk, I'm going to have to wait to post more data. Commented Dec 31, 2018 at 13:12
  • 1
    Please verify your mounts by adding the output of mount to your question. Commented Dec 31, 2018 at 13:22
  • @Hermann I managed to get it. Commented Dec 31, 2018 at 14:18

1 Answer 1

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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
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    Holy cow Tim. Thank you so much. PEBCAK. I knew there was something horrifically stupid I was overlooking. Commented Dec 31, 2018 at 17:44
  • We've all been there, @malan. That's why makes this site (and the other StackExchange sites) so valuable. Commented Dec 31, 2018 at 19:12

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