I formatted one of our Matsui 4gb mp3 players by mistake (!doh!), using a laptop, and received the dreaded 'Boot Error' message on the Matsui, which then stuck.
To fix it try this:-
1 / Remove battery and connect to PC via USB lead, PC should still recognise the player and give you a drive letter
2 / Right click the drive in Windows Explorer and format it using FAT32
3 / copy a few some mp3s to the Matsui
4 / eject the Matsui, disconnect it and replace battery
Ours then worked normally, rebuilding the database and playing as normal.
Rob Davis (Telford UK), December 2007