I have a 2 TB external USB drive formatted to Ext2/3 (can't remember for sure was it 2 or 3, but shouldn't make difference). The Ext filesystem doesn't auto-mount on MorphOS, but I get it easily mounted with the Mounter tool.

I have successfully made backup images of my 240 GB Mac mini and 256 GB PowerBook drives on it under MorphOS using the RAWDISK: device.

So, my suggestion is that you should sacrifice original formatting on some external HDD and format it to Ext2/3. Then it should work in both MorphOS and Linux just fine. I did partition and format my drive under a Linux machine to be sure it'd end up as compatible as possible, but I guess it could be possible under MorphOS too, although I don't remember if I've ever tried it.