Upgrading to Music Manager 2.30 couldn't hurt:
http://www.phatnoise.com/products/software/PMM_2.30_PHATNOISE_WWW.exeLots of things can prevent the program from being able to eject the drive, even just having any other windows open.
Also, with the DMS plugged, right clicking on My Computer, then click Manage, and go to Device Manager. Then find the DMS under Disk Drives, check the properties, and go to Policies to make sure "Optimize for quick removal" is checked.
If you try everything and keep getting the message, you should still be able to transfer music and save it. Try transferring a few tracks, then do the save and eject operation. If you still get "can't eject DMS" message, just close the program and then from Safely Remove Hardware Icon, stop the DMS. I had to do this on another computer, and it still worked fine.