Ok here's what I'd like you to try,
Do you have a PC you can use?
If so install HD Sentinal on the PC then plug the XT in in the external, this will give an easy to read indication of the drives condition,
This is important so we can start isolating where the fault lies,
When you changed the drives did you clone the O/S or load a fresh one?
If it was a clone and HD Sentinal says the drive health is good I would say clean it and start fresh, but only if the drive is in good condition still,
Also have a browse for updates for the Mac as well, in 2008 a 500GB drive should've been supported ok, but there is a chance the Mac needs a bios update as it was designed for HDD's and not a semi SSD's
I've had to advance answer you as I'm not really here anymore so I may not be back again, I only answered this one as I have the newer 750 myself and the company supplied me it, so I felt I should answer the odd one here and there,
As for rma, you will get a refurb unit in exchange, it won't be new, and that's why we must make sure the drive is bad to start with, but it will simply be one like yours that has been repaired and have the latest FW installed, this in itself is a bit hit and miss I'm afraid, in some areas it's all good, others not so good, Asia being one of the worst, so think long and hard about it before you send it in, a simple full format may sort all your problems out, and it must be a full low level format to wipe the SSD portion clean,
So have a look, odds up what is best for you, I'd do the low level anyway to remove my data before sending in, you could do this then a new O/S and if it's ok all is good, if not run it through a secure wipe before sending it back, at least then all your info should be gone and unrecoverable, a 6 pass one should do it ok, hth.
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DOING ANYTHING I HAVE SUGGESTED IS AT YOUR OWN RISK, NEITHER I NOR SEAGATE TAKE ANY RESPONSIBILITY, IT'S YOUR CHOICE TO DO WHAT YOU FEEL IS BEST FOR YOU
