Permission problem with sparsebundle in user share

Ok. I did some additional testing.

Using the chown -R username:users sets the permissions to
drwx--S--- 4 username users 4096 2014-09-17 10:50 macbookpro.sparsebundle

This allows me to open the time machine, but when I want to write to the sparsebundle it tells me that I don't have write permissions (the original problem).

So, just to see what happens, I typed (which I found while reading through a mac forum):

chown -R `id -un`:`id -gn` /c/home/username/macbookpro.sparsebundle

This changed the permissions to
drwx--S--- 4 root root 4096 2014-09-17 10:50 macbookpro.sparsebundle

Then, when I enabled time machine, it started backing up, but by creating a brand new sparsebundle (macbookpro 1.sparsebundle). [sigh]

So, I removed that new sparsebundle, re-chown'd the file permissions to username:users, and am back where I started. Hrm.