Yamaha S90 Owner's Manual page 130

Can't save data to the Memory card .
Is the Memory card being used write protected? (Write-protect should be set to off for saving data.) (Page 82)
Is the Memory card being used properly formatted? (Page 82)
MIDI bulk data transmission/reception won't work properly.
Is the Receive Bulk parameter set to "protect" in the Utility mode? (Ref. #184)
Plug-in board does not work.
Is the lamp of the corresponding slot number to which the Plug-in board has been installed lit? (Pages 73 and 125)
Has the Vocal Harmony Plug-in board been installed to slot 2 or 3? (The VH board should be installed to slot 1.)
(Page 124)
Has the Multi-part Plug-in board been installed to slot 1 or 2? (The Multi-part Plug-in board should be installed to slot 3.)
(Page 124)
S90 hangs up or freezes during start up because of a malfunction or
incorrect operation.
While data is being written to Flash ROM (while an "Executing..." or "Please keep power on" message is shown),
power should never be turned off – this results in loss of all user data and may cause the system to freeze. If this
happens, turn the power off, then simultaneously hold down the [MASTER], [STORE], and [PRE1] buttons while
turning the power on again. If a "System memory crashed" message appears, refer to page 117.
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Troubleshooting