Jammy 4041: I did format the drive as FAT32 for that reason. I did this from the Hardy intallation, as part of installing Hardy.
cyberdork33: After installing Hardy, I had the "no bootable device" error. Synchronising my partition tables in rEFIt solves this. OS X does throw the partition table out of sync every time I try to reformat the fourth partition from OS X disk utility though.
More things that I learnt: When I deleted my 4th partition and reformatted as "no file system", Ubuntu couldn't mount it, but OS X recognised it as FAT (MSDOS) and mounted it. I even mucked around and could write to it.
OS X has not been able to do anything to that partition (format or erase) even when it was "no file system" in Hardy (as above) - only being able to mount and write to it on that occasion.
Vista has not been able to mount the partition or even recognise it as FAT32. This makes me think there is something wrong with the way Hardy is formatting the drive. Apparently I need to download a program for vista to format a partition my size (~100GB) as FAT32. I haven't downloaded this yet so dunno if this will fix the problem.
Good to see you guys are still interested,
I got sick of trying to reformat the partition this way and that - I wiped that partition and the ubuntu partition as well (the one directly before that). I reinstalled ubuntu and labelled the fourth partition as 'no file system' during the installation. I then formatted the drive as FAT32 in OS X. This has worked. I have yet to try read/write in windows, but for now this solution seems satisfactory.
I have one little question now - I cannot figure out how to rename my Ubuntu Hardy partition in OS X - all that will show up is 'UNTITILED'. I have installed the extfsx drivers by the way. This partition shows up as 'File System' in Hardy. Is there a way to rename the drive in Hardy and OS X?