Android :: Computing Free Space On External Card For Installation

My application allow to download files and store them on the external card. To avoid filling it, I've implemented a safety check so it prevents downloading when the free space is less than 20MB. The computation of the free space is done by:

public synchronized long getFreeSpace() { if (statFs == null) { statFs = new StatFs(getExternalStorageDirectory().getAbsolutePath()); }

return statFs.getAvailableBlocks() * statFs.getBlockSize(); }

which I presume is correct. In fact, it works in all my tests and for what I can say for the 99% of users. But a single user notified me that my app is constantly giving the "card full" message, as if the above method didn't work. He also told me that the problem went away after a while, and returned after upgrading a new release of my app. The user seems an advanced one, and I think he's able to check and correctly manage the contents of his external card. He reported that the card has more than 1.4GB free on the card. Are you aware of any specific issue or bug? The smartphone of the user is a Dell Streak with Android 1.6.