Windows 8.1 creates a directory and file "\System Volume Information\IndexerVolumeGuid". (The folder is hidden, but you can see the file by opening your card in Windows Explorer and manually entering the directory in the location bar.)
Their presence seems to confuse the ResMed and some other equipment. If you forget to lock the card before using it with Windows 8.1 and your ResMed wants to reformat the card, choose "No" and put the card back in your computer (still unlocked).
You can remove the directory and file from the command prompt (or a batch file) with the following commands (for an SD card in drive F: in this example):
del "F:\System Volume Information\IndexerVolumeGuid"
rmdir "F:\System Volume Information"
It should then be accepted by the ResMed without reformatting.