This sometimes happens when there is a drive conflict when attaching external storage (which is what the R4 is when attached to a PC). Although, considering it happened on 2 computers that might not be the problem. Right click My Computer on the desktop, choose Manage, then click on Disk Management on the left. It's possible that the drive will show up here, but isn't getting assigned a drive letter. If true, then right click the R4's drive on the bottom part of the screen and choose Assign Drive Letter or something like that. Then pick a drive letter that isn't being used. If that was your problem, once you do this, it'll never be an issue again. BTW, this is a handy trick for any external storage that you might be having trouble with, including USB thumb drives.