How to lock a file against deletion but still make it writeable?
For Windows:
- Deny "Delete" permission on the file.
- Remove or deny "Delete child items" permission on the parent directory.
For Unix (including OS X):
- Remove "Write" permission on the parent directory.
Note that this will only prevent the file from being removed (deleted), but won't do anything against accidental truncation or overwriting with garbage. If a file is writable, you can write anything to it, period.
Also, file permissions are next to impossible to translate between operating systems. If the NAS runs Linux, and you try to set permissions from within Windows, the end result may be different from what you expect.
In Linux you could create a hard link to it. Then you can write to it and "delete" it, but you'll be only removing the reference in your directory. The other hard-link will still point to the file's contents, so it won't have been deleted anyway.
In Unix world, you don't "delete" files. You just decrease the number of hard links to it. When nothing else is pointing to it, the space is considered free and can be used…
Backups. You can't really protect a writeable file from damage even if you can from deletion. Back it up daily.