"Write Error: No space left on device" when df displays 60% of free space? [duplicate]

It's been happening for the last few weeks in a small unix server (it doesn't have all standard unix commands, just the basics included in the BusyBox package) that I was configuring to backup files.

If we check the free space of the server it shows:

 Filesystem             |   Size  |    Used | Available | Use%| Mounted on
 /dev/hdc2              | 678.4G  |  416.6G |   261.8G  | 61% | /tmp/mnt/D
 /dev/sda1              |  96.1G  |   36.0G |    55.2G  | 40% | /tmp/mnt/USB/USB_C

Now I am trying to copy either a 700MB files to USB_C or a 90GB files to /tmp/mnt/D where I can see that there's plenty of space available, however, I keep on receiving the followin error:

cp: Write Error: No space left on device

Any idea what the problem could be and how it could be fixed?

As mentioned, this is a small unix server with the bare minimum of unix commands, so most of the commands mentioned in other posts do NOT work for this box. Hence that I might need the help of someone who knows what I could try in this specific system to give us some clue...

My gut reaction says inodes

Check your inodes. Those are the hidden part of the filesystem that keeps metadata about the files.

df -i

I suspect you may have many, many small files. If that's the case -- and note that each file needs an inode -- you can use up inodes before you use up actual storage space.