The Mac 800K floppies are, as I suspect you know, a totally different drive mechanism from the generic 800K PC external floppy disk drive, even one confirmed working for Mac and connecting over USB.
It was a Steve Wozniak invention, it used Constant Linear Velocity (ie. the floppy spin rate reduced as you read the outside edges of the surface. This meant that it could store more data, like 774KB instead of 720KB on a PC 800K floppy. PC floppy drives use an alternative Constant Angular Velocity (constant spin rate).

As far as I know, it was only Apple drives that did this. Then came the 1.4MB Superdrives, they switched from 80TPI (tracks per inch) to 135TPI (how do I remember that?) and again, in the Mac, when reading an 800K floppy it used CLV. For 1.4MB floppies in Mac, it used the PC CAV, for compatibiliy I think.

So, in summary, I think you need an original Apple floppy drive, either external or in an old Mac, to read these disks. Like you, I have my old SE to let me image any floppies I come across (like Corruption), but many years ago, before I gave away my IIci, I imaged every floppy I had, about 240MB worth - and that seems to be the best approach, image it onto hard disk while you can. Only a few years behind on the same task with all my magazine cover CDs and DVDs - all imaged and stored on a 2TB disk. If I ever need access to a file it's no more than 2 clicks away.
Suppose I've also done the same with my entire (rather large) DVD collection - all ripped into iTunes movies and TV shows - I never have to pull out a DVD to watch it, it's all spread across a few TB disks in the Mac Pro.

While it doesn't at the moment, OmniFlop may be able to deal with 800k Mac floppies in the near-ish future. Right now it can only read 1.44mb Mac disks. It's a truly remarkable program - it can read and write just about any format floppy in a Win32 environment using the standard drive and controller. I've used it before with BBC Micro disks.

Nope, you'll need a Mac floppy drive. 400K and 800K disks use variable-speed recording to pack more data on the disk.

The external drive requires a Mac with a floppy port. The floppy controller chip is the important part here. A PC with either a Kryoflux or a DiscFerret device would be able to read the data, but as far as I know there is no utility to convert the images into proper DiskCopy 4.2 images, or another usable format which preserves tags.

Please do NOT use Disk Copy 6 for imaging floppies, DiskCopy 4.2 preserves more data.

I use a Quadra 650 to read the disks, then either copy the files to an iMac G3 using AppleShare or make Disk Copy image and upload it to my desktop PC using Fetch and FileZilla.

Unfortunately I don't think there's an easy way to read single- and double-sided Mac floppies in a standard PC floppy drive. I haven't tried, but a disk imaging tool like WinImage, RAWREAD, or DD might be able to create a disk image if the drive can track the disk properly.

I haven't tried, but a disk imaging tool like WinImage, RAWREAD, or DD might be able to create a disk image if the drive can track the disk properly.

Nope, you'd need a controller card that can either write at an extremely high sampling rate, or a variable-speed drive.

Well so much for that idea.

There is a USB floppy controller out there marketed for backing up floppies called 'Cryoflux', which uses standard PC drives and custom software to dump floppies. Apparently it does successfully dump 800k floppies to images that can mount on Mini vMac, but it's expensive ($120 for the base model) and is out of stock with no ETA on a new batch.

http://kryoflux.com/
http://68kmla.org/forums/viewtopic.php?f=7&t=15294&p=146012#p146012

Yeah, when you can buy the old Mac with floppy drive for $15. A standard PC floppy drive uses CAV, you need a Mac drive with CLV - for reading 800KB floppies.

There's the Kryoflux, as well as the DiscFerret http://www.discferret.com/ but both of these are intended for people who know what they're doing. They have the great advantage of being able to back up copy protected disks, with copy protection intact. However, the software for them is not at all complete.

If you can get your hands on either a PC or a Commodore Amiga you can also try to get a Catweasel Controller from Jens Schönfeld: http://www.icomp.de/home/indexe.htm
Then go to "products" then "PC/industriell" and there you will find Catweasel ISA and Catweasel MK4 PCI.
You can also find both on ebay sometimes. The ISA version will cost 39,90 € and the PCI version 99,-€ + shipment. The actual list of supported formats is the follwing:

* all PC-formats (180K up to 1440K)
* Amiga DD and HD (also 5,25" formats)
* Atari 9, 10 and 11 sektor disks
* Macintosh 720K, 800K, 1440K (DD, GCR, HD)
* Commodore 1541, 1571, 1581 (C64, C128 and 3,5" C-64 disks)
* XTRA High density with 2380KByte per disk
* Nintendo backup station 1600KB format
* Atari 800XL (all MFM formats, FM under developement)
* Apple IIe disks (Apple DOS 3.3 and up)
* further 8-bit formats planned

So provided you have the drive you can read all this in a PC at least (sadly there is no support for Apple 400k disks). Pay attention as the PCI variant is no longer in production and there is only a small stock of it left (I don't know if there is a new production run planned somewhere in the future).

Please don't buy the Catweasel; compared to the Discferret and the Kryoflux, it's pretty lousy, and it hasn't been improved for years.

For people who don't want to play around with it on their own, I recommend the Kryoflux since it's the easiest to use. The Discferret is arguably more powerful, but the software isn't nearly as developed.

Again, I cannot recommend the Catweasel. It was good, but now we have better things.