Many older computers recorded on only one side of the disk. So, people would fill one side of the disk and then flip it over to store more on the other side. Disks used this way are called "flippy" disks.
5.25" disks have a hole, called the index hole, that lets the drive know if the disk is rotating. (The index hole has other purposes also.) The problem with flippy disks is that when the disk is inserted upside-down, the drive cannot see the index hole. Many drives won't read from the disk unless they can see the index hole. If you have one of these drives, the FC5025 will be able to read from the first side of the disk only. When you flip the disk over to read the other side, the drive will not send any data to the FC5025, and the FC5025 will not be able to read that side.
Please note: even the recommended TEAC FD-55GFR drive cannot read both sides of flippy disks. There is no recommended drive for reading flippy disks at this time.