I use external drives to hold all my Poser stuff (not the program) and have had two that failed.  The first one was a Maxtor (now seagate) which was under warranty for one year for replacement but not for Data recovery. That cost me $300.00 which was cheap compared to most of the places I checked into and the second was a 500 gb Western Digital I BOOK.  I had the latter for only two weeks and after loading all the stuff I bought from Daz It craped out.  Lost everything, but fortunately Daz has a re-set option and I am slowly getting what I can back other then some of the freebies which aren't available anymore.  With Western Digital there is also a 1 year warranty but you must use their authorized Data recovery services as not to void the warranty  and when I called them, I was told it would be a miminum of $1,000.00 to get the lost data back so I told them where to go.  If there is anything I can say to help anyone out there.......it is to Recommend Not to buy Western Digital and to back everything up on disks.  In fact that was my intention when I bought the 500 gb drive in order to get my stuff organized to put on disks. 
Maxtor was great and allowed me to take the hard-drive where-ever I wanted as long as they got a letter from the Data recovery service stating that it was opened by them to recover the data and they sent me a replacement "Free of Charge!"
Western Digital was less then helpful and in fact they sent me an e-mail informing me that "They had shipped the wrong cables" with the hard-drive, warning me that it could damage the hard-drive if I used it, two weeks after the hard-drive crashed!!
I took the drive back to the store where I bought it and got my money back, but I am still working on getting my Daz purchases back.  It's really more a pain and time spent downloading stuff I had but I was lucky that I could at least do that. 
Personally, I would not trust running Poser on an external hard-drive unless you have what you want on a back-up disk.  You never know when it may crap out and you may be left with nothing.