> For example, ext4 is officially supported read-only but in my case it didn't read some folders properly.
For some reason, I've thought ext4 is "pervasive" or "fundamental" until now. I assumed it to be readable by most systems. So it came as a surprise that OpenBSD could not correctly read a ext4 filesystem. But thinking again, last time I checked, Linux could not write to a HFS+ filesystem, either. OpenBSD's FFS might also not be not supported. So a BSD not supporting the Linux filesystem is very natural.
Probably one of the "greatest common divisor" filesystems, which are supported by all major operating systems, should be FAT32. Which is a shame, as it isn't neither an open standard nor a thing from the Unix culture. It also lacks journaling support, which I consider essential. Any alternatives?
FAT32 is an open standard AFAIK, however it's extensions (eg long file names) and forks (eg exFAT) are patented. But in fairness, FAT32 isn't much use without support for long file names.
Interestingly, some FAT32 forks do support journalling. Sadly those tend to be the patents Microsoft are the most proactive in upholding.
As for an alternative, ZFS is supported on FreeBSD, Linux, Solaris and OS X - so that's one option. Albeit it's not a great option in this specific criteria. ext2 receives pretty good support as well, even on Windows. However ext2 doesn't support journalling. Another option is to run ext3 as that gracefully downgrades to ext2 if no ext3 driver is available. Sadly NTFS is probably the most ubiquitous journalling file system. ntfs-3g - which are actually pretty decent drivers - has been ported to quite a few platforms.
UDF. It was made for optical media, but works just as well on hard drives. About the only caveat I've found is that you have to be pretty particular regarding what format you choose, but I've found that Windows' format for UDF tends to make the most compatible disks.
For some reason, I've thought ext4 is "pervasive" or "fundamental" until now. I assumed it to be readable by most systems. So it came as a surprise that OpenBSD could not correctly read a ext4 filesystem. But thinking again, last time I checked, Linux could not write to a HFS+ filesystem, either. OpenBSD's FFS might also not be not supported. So a BSD not supporting the Linux filesystem is very natural.
Probably one of the "greatest common divisor" filesystems, which are supported by all major operating systems, should be FAT32. Which is a shame, as it isn't neither an open standard nor a thing from the Unix culture. It also lacks journaling support, which I consider essential. Any alternatives?