From my personal notes:
ProApp 20
The unusual ProApp 20 drive is an external device with both SCSI and floppy port connectors. Macs can connect to the drive via either port and Apple II computers can use the floppy port (the ProApp predates the availability of Apple II SCSI controllers). An external power supply is used.
Software supplied with the drive can partition the disk in separate volumes for the Mac and Apple II. It is also possible to connect a Mac and an Apple II to the disk simultaneously, but an I/O error occurs if both try to access the disk at the same time, limiting the use of this feature. If used with a Mac on its own, the disk can be formatted using a standard SCSI formatter. The drive cannot be included in a SCSI chain (ie it must be the only SCSI device attached to the Mac).
ProApp 20
The unusual ProApp 20 drive is an external device with both SCSI and floppy port connectors. Macs can connect to the drive via either port and Apple II computers can use the floppy port (the ProApp predates the availability of Apple II SCSI controllers). An external power supply is used.
Software supplied with the drive can partition the disk in separate volumes for the Mac and Apple II. It is also possible to connect a Mac and an Apple II to the disk simultaneously, but an I/O error occurs if both try to access the disk at the same time, limiting the use of this feature. If used with a Mac on its own, the disk can be formatted using a standard SCSI formatter. The drive cannot be included in a SCSI chain (ie it must be the only SCSI device attached to the Mac).