I've tried a Samsung that's purely USB and a hefty old LG which has mains power. Though they show up in both Raspbian and Xbian the DVDs don't play. I've installed the paid for codecs. Presumably the Samsung won't work unless connected to a powered hub and then only if the missing factor is dealt with, whatever it is. Both drives still work with other computers.
I'm using the latest Xbian and Raspbian, both otherwise working great with a Sandisk Extreme class 10 16gb SDHC card. In the case of Raspbian I also added other codecs just in case. If I click on the DVD as named in XBMC instead of showing the contents or starting I get a list of the root filesystem folders.
As 4OD and iPlayer were already working I'd bought the MPG2 codec thinking that would make DVDs work.
I have a Samsung 128gb SSD which makes the Pi crash and reboot instantly on connection. (The first time I tried it plugged in before powering up Xbian got stuck at the flashing cursor but it's doing fine now that way.) I connect it through the cable it came with which has the SATA connectors at one end and USB at the other. USB memory sticks have been fine. I'll try the SSD in my enclosure with another cable later but is there any obvious reason why this is happening or a wrong setting I have?
Apart from my other current enquiry here I think the Pi is outstanding and underpriced (forego two takeaways and it's paid for!), just want to iron these couple of things out as I'm not intending to get experimental with it myself.