Currently Being ModeratedNov 16, 2012 10:39 AM (in response to Nick Kutzko)
Great news, thanks Nick... I see what you mean & didn't really think of that angle.
Ahhh, here it is...
Can external USB, FireWire, or Thunderbolt hard drives be added to Fusion Drive?
An external drive cannot be used as part of a Fusion Drive volume. Fusion Drive is designed to work with an internal hard disk drive and internal flash storage.
Currently Being ModeratedNov 16, 2012 10:44 AM (in response to BDAqua)
Actually, what they're saying is NOT true as a blogger did just that using command line tools.
I just want to be sure it's not going to cache open files from other drives on its own.
Currently Being ModeratedDec 11, 2012 7:12 PM (in response to BDAqua)
yes indeed an external drive can well be used as part of a fusion drive. I have done it myself and both drive must be connected at the same time to work. I have sucessfully combined 240 gb ssd with my 2T external hard drive using usb 3.0 ports.
Currently Being ModeratedDec 10, 2013 9:36 AM (in response to Nick Kutzko)
You can definitely create a Fusion Drive using an external drive to add capacity, and in fact it can be a great thing to do as the SSD will eliminate most of the latency involved.
The SSD will contain whatever the most frequently used blocks are, and these will moved from your HDD(s) onto the SSD to provide the performance improvement. This means you could potentially end up with any number of your working files on the SSD depending upon how often you use them (or have been using them recently). This is important to keep in mind; the SSD in a Fusion Drive is not the cache as a cache-disk, meaning that if the SSD fails you will lose data and have to rebuild your Fusion Drive from backups.
This will essentially eliminate the redundancy advantage of RAID-5, as while you can replace any HDDs that fail as normal, an SSD failure will render the volume unusable.
I'm actually currently interested in whether a RAID-1 of SSDs can be used within a Fusion Drive, as it could allow the SSD part of the Fusion Drive to be given redundancy too, in which case the setup would be safe as it'd be 2x SSDs in RAID-1 + HDDs in RAID-5, giving you one to two disks worth of redundancy. However I suspect it won't work, as I believe Fusion Drive just queries each disk to determine if it is an SSD, and of course an Apple RAID won't identify itself as such.