In busybox 1.25.1 df command ends with error if device have long mounting point name with spaces (example "Seagate Backup Plus Drive".
Then have result:
# df -h /dev/sdb1
Filesystem                Size      Used Available Use% Mounted on
df: /tmp/mnt/Seagate\040Backup\040Plus\040Drive: No such file or directory

and:

df -h /tmp/mnt/Seagate\ Backup\ Plus\ Drive/
Filesystem                Size      Used Available Use% Mounted on
df: /tmp/mnt/Seagate\040Backup\040Plus\040Drive: No such file or directory

For comparison, ls command read the device content:
# ls -l /tmp/mnt/Seagate\ Backup\ Plus\ Drive/
-rwxrwxr-x    1 root     www             32 Jul 17  2012 Autorun.inf
-rwxrwxr-x    2 root     www         562718 Aug 15  2013 BackupPlus.ico
...

If make mounting point name short, then df command work.