My Eris works fine when I mount the SD card after plugging in, so it should work.
I have used Missing Sync for Windows Mobile in the past. I was never happy with it; I kept having issue with it not syncing calendar events properly - it would get a cryptic error message that you could never trace to a particular calendar event, but it would prevent calendar syncing completely. I finally just went to syncing Address Book and my calendars using Spanning Sync on the Mac (Busy Sync can also sync calendars, and Address Book in Snow Leopard also syncs with your Google account.) I'd really suggest doing this and, if you wish to back up SMS messages (something that Missing Sync also offers), look into SMS Backup in the Market (if it works with 2.1.) It can backup your sent and received SMS messages to an archived label in your gmail account.
For media syncing, DoubleTwist on the Mac is an excellent choice (though, of course, you need to fix that pesky non-mounting problem.)