Well, I've just received my next last week, it was stuck in paris until the postal service decided to send by boat 22 tons of parcels waiting there since last June.
Anyway, I'll have to find some documentation about the .mdr file structure and grab a sample. The specificity is it contains a drive and not only a single file, but the files inside should be stored in a straightforward manner, with, I suppose, a classical header as they were used in the original microdrive files.
The first and easier thing will be to parse then using a PC and extract each file, but to make usable, I guess it will be necessary to convert them into .bas, .bin, etc. files with a header that would be suitable for the NextOS.
Then it should also be possible to directly write a tool running on the Next and opening the .mdr container, parsing the files and writing them back using the integrated SAVE command.
Well, it looks I have lots of file formats to study when I have some free time, and since I need to get my original work back from my old microdrives and +D disks, that's somehting I'll have to do anyway. I also have a Vdrive ZX but I hadn't found the time (nor free desk space to put them back to work :p) to plug it into my old 48K and IF1 yet, but transferring the contentes of the (hopefully) still readable microdrives to a Vdrive ZX SD card should be a priority... At least to backup everything that can be.