Hello Don,
This AP80EX appears to be an external 80 MB hard drive. With a 25-pin connector, it would be SCSI.
The floppy drive of a Macintosh Plus can only work with up to 800K DSDD/2DD floppies (not 1.44 MB HD disks).
http://support.apple.com/kb/sp190
Please note that a USB floppy drive is not compatible with the special Macintosh 800K format (only 720K PC and 1.44 MB Mac/PC would be OK).
So, a USB floppy drive would not let you read any old 400K/800K Mac floppies. In such a case, an intermediate machine is necessary, and you seem to have one, the Macintosh Classic (I assume that it is the "real" Macintosh Classic, not just any "classic" Mac).
http://support.apple.com/kb/sp198
You should be able to connect any external SCSI hard drive to either the Macintosh Plus or the Macintosh Classic. If you use the Macintosh Classic, you could then copy files to 1.44 MB Mac-formatted (HD) floppy disks without a problem. With system software 7.0.1 or 7.1 on the Macintosh Classic, you may even be in a position to write to PC-formatted 1.44 MB floppies via an application called Apple File Exchange (System 7.5 would have a PC Exchange control panel).
1.44 MB floppies could later be read by a USB floppy drive on a modern computer. You could perhaps try the Sabrent floppy drive with another machine (Mac or PC) if it does not work with the Mac Pro. Also, some semi-old PCs may be equipped with an internal floppy drive. However, to read a Mac-formatted (1.44 MB) floppy at a Windows PC, a Mac-disk utility such as MacDrive or TransMac (an appropriate version that still supports floppies) would be needed.
There are other transfer possibilities. For example, one can set up a null-modem link between a Macintosh Plus or Macintosh Classic and more modern computers (Mac or PC). If you would be interested in details about cables, serial adapters and software, please post back.
Jan