Recently, I acquired a kryoflux board to help read in the Venix floppies... I finally have it up and running.
I've managed to read in most of my collection of ~300 Rainbow floppies. 95% of them were actual Rainbow disks. The other 5% were either completely blank, or some variety of IBM format (including 3.5" 720k on a 5.25" floppy!) with a couple of 'robin' formatted ones just to keep you on your toes. I didn't wind up seeing any of the very rate dual-sided RX-50's (they exist, but are super rare because this was a home-brew job not an official thing).
While it is possible just to do
dtc -ffoo.img -i4
It takes a long time since it's reading the back-side that has no data on it. Using
dtc -ffoo.img -g0 -i4
is all that's needed to go fast. Some caveats should be observed.
Nearly every track has extra data in at least one of the sectors. This threw me for a loop, but is harmless (as stated in the manual).
I had some issues with old, ill-maintained floppy drives. Bought a refurbished one and all my troubles just went away. I had several old drives and based on the good floppy drive, I could find known good floppy diskettes and was able to refurbish one of my old units. I had the best luck with the TEAC FD55-GFR drives, btw. Not sure it's worth while to refurb the rest of the units, though, since I have something that's working...
Keep an eye on the number of sectors reported. It's usually 10. If it isn't, stop right away. It isn't a Rainbow / RX-50 formatted disk. If it's 8 or 9 sectors, it's likely an 320k or 360k IBM floppy. Removing -g0 and adding -k2 is useful for these diskettes (saves time more than anything. If it's 15 sectors, it's likely a 1.2MB floppy. Had 2 or 3 of those mixed in with these diskettes despite never having a machine that produced them.
As these disks age, you may need to try multiple times. I have some disks, though (maybe 1%) that I can't read back certain tracks on even with that. The default is good, but trying more helps with a similar number (so -t10 -tc5). I don't know if there's a way to retry individual sectors, or if dtc is internally buffering good sectors from previous retires or not. Previous programs I've had on the Rainbow for extraction had great luck retrying sectors multiple times when the full track read didn't work...
Some disks are copy protected. Not sure how best to preserve these disks. The -i4 format won't work with, for example, Lotus 1-2-3 system disks because tracks 78 and 79 have sectors numbered up to 12 and are missing sectors (they do this with a special format that I believe omits sectors 9 and 10 and instead has sectors 11 and 12). There are other copy protected disks that have deleted sectors since the Rainbow could easily read those, but they were hard to create from standard interfaces. Don't know if -i2 images would help, or if I have to go all the way down to -i0 to ensure proper imaging. The copy protected disks, however, were all broken in the '80s, so preserving them isn't so interesting to me. There's also a couple of disks that have data on track 82, but it's marked as track 79 and it's unclear what this data is. I know there are several old formats that preserve this info (TELDISK and DISKIMAGE being two that I've run into most).
cpmtools is able to extract data from CP/M formatted disks (use dec_pro format). It understands all about the interleaving you need to do on tracks 2+ and can work on raw kyroflux images.
As for MS-DOS disks, I wrote a small utility to convert the physical layout to a logical one, and to (optionally) replace sector 0 with a standard boot record with the BPB in it (Rainbow floppies have Z80 boot code which has a different starting byte (f3) and doesn't have the tables expected. Doing this, both mtools and FreeBSD's msdosfs can copy files easily enough. But so far it's not reversible, so you can't (yet) use it to create floppy images. I'll be testing two tricks (just prepending the magic sector and appending sector 0 at the end) to see if that might help. I can post a pointer if there's interest.
Venix disks are a mixed bag as far as data extraction, but are otherwise boring: 80 tracks, 1 side, 10 sectors per track. Just had brief access to the distribution diskettes, mostly prior to having kryoflux (and the reason for buying it). Ironically, I couldn't get the kryoflux working due to the drive issues I discussed above before I had to return the distribution diskettes. But I was able to copy them natively on my Rainbow...
Finally, there's still 2 or 3 disks that I've been able to read in that I have no clue what the format is. They are kinda sorta CP/M, but also kinda-sorta MS-DOS from looking at the first two tracks, but neither quite works to decode them. Physically, they RX-50. They don't look like ODS-2 nor v7 unix file systems, but that's harder to know for sure. They are unbootable (tracks 0 and 1 are 0xe5, the erasure pattern for RX-50s).
My only real complaint is that it would be nice if dtc would spit out a summary at the end, including tracks unreadable. I have to keep an eye on the logs otherwise. Not a huge deal, to be sure.