The solid state drives mount as a USB mass storage device ( eg. mount /dev/sdx1 your-mount-point), and uses a FAT filesystem. Adding music to the rio is as simple as copying MP3's to the drive. The only catch is that the MP3's must be contained somewhere under the Music sub directory. There doesn't seem to be a limit as to how many sub directories you use to organize your MP3s on the device (I have used 2 sub levels so far).

I am using the Rio with an extended 512MB SD card for extra storage space. The memory card mounts as its own drive under Linux, which works well. I would prefer one spanned partition between the internal and expanstion "drives", but I can understand how this would cause some problems if the card was removed. It works well.

Rio supports playlists, which are standard .m3u files, which is nice. I store my MP3s in the following directory structure:
band_name/album_name/
so I use the following command to make playlists of each band (from the Music directory):
find band_name > band_name.m3u

I have not had the device long enough to comment on the battery life (advertised as 18hrs for a single Engergizer AAA). However, I have used the device for 10hrs, much of which involved transfering files, and the battery indicator is still +50%, so it does appear to be working as advertised.

I only have 2 complaints so far with the Rio. The first is that when MP3's encoded at 56KB/s are played, the Rio skips the last 9 seconds of the file. The second complaint is that the headphones included with the device are not the best. They are the type that hook onto your ears, which have a tendancy to come loose. Since this is my first pair of headphones of this type it may just be something that I need to get used to. The headphones do sound good, so its not all bad.

The Rio Cali is a decent MP3 player, but the compability is almost non-existant.

The kernel detects the Rio through the USB interface, but does not associate it with a device node (i.e. /dev/sdb). Fortunately, one can make use of a third party driver, rioutil. This allows you to transfer files on the Rio via the command line, which is hardly ideal. Other users claim to have gotten the Rio to work as a mass storage device, but I have had no such luck.

What really pisses me off about the Rio is the lack of support for the SD card. Music must be transfered onto the SD card using their own propietary software. I imagine that if you upload it into a special directory that the Rio reads from, it might work, but I haven't been able to determine this hierarchy yet. Perhaps if I get access to a Windows computer, I might have a shot.

Tech support was absolutely useless. They told me they only support Windows and Mac. The lady claimed she doesn't even know what Linux is. This is unexcusable for any company, so based on principle alone, I would not recommend someone to purchas from Rio.

It's a shame, because the hardware looks fine. I don't expect every company to go out of their way to support Linux, but Rio seems to go out of their way to discourage Linux. Especially with the SD card.