Congrats on your new shiny!
The way that Mango works (in both Mango OS and android) is that it uses an "on the fly" database. It is not quite like Neutron or UAPP that scan, then store the database. If you take the card out before powering down the device Mango WILL lose connection with the source even though it retains the database. Either that, or your card (or network source) has failed. I am not sure if the app has to mount the source (and stay connected to it) separately from the OS or what, but it seems like that, whereas UAPP and Neutron just create a simple database file and only use that. The point is that even though it retains the database of songs you scanned, it loses connection with the source and the only way to fix it is to rescan the library again.
This happens to me sometimes because I use my 1TB card in both the DX300 and the DX160 and sometimes forget to power down before switching cards (even if the app is closed). This doesn't happen with the other aforementioned music player apps. BTW! This is also another reason I don't use Mango anymore, because it seems as though I have to rescan my library quite often, and it takes quite a while with 30K songs.
If your music source is still good (be it micro-SD card or network source), you SHOULD be able to simply rescan your library.
I must have been having a brain fart when I said earlier that the MAX and edition X were the same internally.... I know better than that... Maybe I was thinking about the Edition X and the regular DX320, which ARE indeed the same internally. Apologies if I helped in misleading anyone.
I also should have noticed the 500 units claim because I remember making a comment about 888 being an odd number, and
@jamato8 informed me of cultural meanings that might be behind the overall numbers being released.