🚀 go-pugleaf

I added an external (toshiba) disc to my 520stfm recently and
have seen  this also.  It seems the OS selects each disc and samples 
the WRITE PROTECT during vertical sync periods.  If it detects the
WP to be asserted anytime then it assumes that a disc change had
occured.  So swapping in a protected disc is always detected.
Before you get around to access the disc, lots of samples 
have come back with the WP true.  Now what about swapping a
writeable disc for another writeable disc?  This is probably
the case where the new disc was not detected.  So what happened?
Didn't the WP go true when the disc was removed?  The drive only
has to appear write protected momentarialy and the write protect
switch on the drive is in the same position it would be for many
samples during the time after the old disc is removed until the the      
time that the new disc is inserted.  Well there's the problem.  Most 
drives I've seen will not assert WP if there is not disc in the drive 
so you don't get the writeable to write protected to writeable 
transitions in the WP line you might expect.        

I don't have a specific fix for you disc but if you think your problem
caused by what I described above then you might try the following.
If your drive has a DISC CHANGE signal on it (usually pin 2 sometimes 
pin 34) you can hook this line to your WRITE PROTECT line.  This will
solve you problem of recognizing a new disc but this can lead to
other problems on some drives (in my case writeable discs sometimes
appeared protected and the fix is another long story).