Is your computer a Mac by any chance? Mac computers will sometimes add a small file to the SD card when the SD card is removed...unsure if it is when SD card is removed improperly or not. It is discussed in the SleepyHead user guide.
A few people have found out that a change to the SD card has happened by accident when Windows may have added a small file.
The problem is that the ResMed machine is real nit picky about the files on the SD card. One tiny change or something being there that shouldn't be there will cause the machine to reject the SD card.
If you have a new blank generic SD card you can use it and the machine won't have a problem.
You could manually erase the data off the SD card and then insert the card and the machine will write the data that is stored on the computer over onto the blank SD card.
I suspect that some little something has happened to the file structure on the SD card and the machine says "whoa, this isn't like it is supposed to be"....it won't take the SD card as it is...so either use another SD card or manually erase the SD card and use it.
Any data that your DME will want is stored on the machine and will be put back onto any blank SD card so you won't lose anything important. Some minor hi def detail data will be lost because it isn't stored on the computer but you have it in the software. You only have a couple of nights of data anyway.
You need to try to figure out what caused the SD card files to get messed up though. SleepyHead doesn't do it.
There is nothing in SleepyHead that will alter the SD card contents.
Improper removal of SD cards might mess up a file...accidentally adding or removing a file to the SD card will change what the machine expects to see and it will reject the card. It is real nit picky.
Sliding the write protect slider thing to "read only" will prevent any changes from being made to the card.
Though I use a ResMed machine and since Feb I haven't had any problems and I never bother to write protect the SD card.
Okay, I see not a Mac...other ideas are the same...something has caused the file structure to change.
If you are good at locating files you might see if you can sort through the SD card files and find one that was added or modified about the time you used the software...try removing it to see if that fixes things.
Otherwise...use a different SD card (generic works fine) or erase the card using your computer and start with a blank card. The machine stores enough data that no real harm is done and it will put the data that is stored on the machine back to a blank card.