External floppy drives on the Spectrum +3
To connect a normal PC floppy drive as "drive B:" to the +3 you need a ribbon cable with an edge connector at one end, and an IDC connector at the other.
Modern floppy drives don't provide the old "Ready" signal, so the ready line needs shorting to ground to signal to the spectrum that the drive is ready (adding a switch here is a good idea otherwise the spectrum won't start up unless there's a disk in the drive).
Any modern floppy drive will work as drive B: with the above proviso, you must create a ready signal for the spectrum otherwise it won't work. All modern drives are hard wired to appear as drive B: (hence the "twist" cable in a PC to make it appear as drive A:) so the cable is just a straight through wiring of all the signals.
The +3 doesn't know about High Density (1.44MB) diskettes so you need to use Double Density (720K) ones, aka "blue floppies". These can be bought online, I don't know if they're still being manufactured anywhere or just new old stock. You can also cheat and tape over the hole on a HD diskette to make it appear as a DD one. This may or may not work, and the data may not stay forever (or it might be fine! but better safe than sorry if you save anything vaguely important on them). If you're just going to use a single diskette to "ferry" files from a PC to 3" disks then this is a perfectly adequate solution.
There are various software tools to write DSK images to physical diskettes, and individual files to +3 formatted diskettes. The other option (if you just want to "ferry" files from the PC to the +3 and save them on drive A:) is to format the disks to 720k DOS format and use a tool on the +3 to copy the files from the DOS floppy to the +3 format floppy in drive A: (or to the ram disk and then back onto a +3 formatted 3.5" disk in B:)
to access +3 formatted disks and format disks to this format on Windows NT you will need the FDrawcmd driver by simon owen. This will only work with real floppy drives, not USB floppy drives attached to laptops. A good utility for writing the images is John Elliot's dsktool.
Writing individual files
To write individual files to a +3 formatted diskette (or dsk file) you can use cpcxfs
DOS format disks
to access a DOS formatted 720k disk on the +3 you can use Garry Lancaster's MSDOS utility This will allow you to "log on" to a DOS format disk in drive B: and copy files from it to drive A: