Windows 7 - how to add files to cd-rw

Hi all,
For some reason when using windows 7 inbuilt burner I can no longer drag and drop a file to burn to a CD-RW disc which has other files on it, just get a message to insert a suitable disc! I know it used to work so don't have a clue why not now. I have set up imgburn as default but trying to use that gives me no option other than format the disc prior to adding the file. Any help welcome txs

.
CD-RWs are notoriously unreliable in my experience. If you can read the disk I recommend you backup the existing files. Then try another disk to see if it's a media problem.

CD-RW media made sense back in the day when optical media was very expensive, but CD-+R is very cheap now and much more reliable.

In my opinion, your CDROM only in CD-R mode only not RW
Maybe your CDROM need additional driver or not compatible with Windows 7
CMIIW ..

Thanks for your replies,
I have found out it was actually imgburn that is not able to add to a part full disc and was set as my default. I changed to ashampoo, which I had previously used, and walaaa, all OK
Victek, interesting comment about CD-RW's being unreliable. I had never heard that before but yes, I have had trouble at odd times with them. Guess you learn something every day!