First and foremost, in bold to make it crystal clear:I would not do ANYTHING with that 750GB disk until some questions can be answered first. DO NOT HOOK IT UP TO AN ENCLOSURE, NOR A WINDOWS PC. If it is your only copy of your data, DO NOT DO ANYTHING WITH IT at this time.
The HP MediaSmart EX485 is what looks to be a consumer-grade NAS: »h10025.www1.hp.com/ewfrf/wc/prod···=3855978
This means you've bought a "black box" product and how it functions (its filesystems, how it stores metadata, what RAID model it uses (possibly something proprietary), etc.) is entirely known to only one company: HP.
You had a 1TB drive and a 750GB drive installed on the system, and if those made up "one volume", then effectively some form of RAID (or RAID-like) is in use. You need to give more details about that configuration. Give as much detail as possible.
Now, that's just the disk configuration. Next we have what actual filesystem it uses -- do you know what filesystem this black-box product uses? Does it run Windows? Does it use NTFS? Does it run Linux? Does it use ext2, ext3, ext4, reiserfs? Again: unless it's in the product documentation, the only people who know this is HP.
This is why I often warn people of buying products like this -- if you purchase these kinds of solutions, you need a support contract with the company who makes it, because they're the only ones who know how the device operates/behaves/etc..
Is it *potentially* possible to get your data off that disk? Yes, but again, without knowing if RAID was in use, and without knowing what filesystem the NAS chooses to use for its storage, it's very hard to say. Again: if this is the only copy of your data that you have, do not stick it in a system "to find out".
The safest thing I can think of would be to hook the disk up to a FreeBSD or Linux-based PC natively via SATA (DO NOT
use a USB enclosure or anything of that nature; all this will do is make troubleshooting worse) and do some actual data forensics to see if the data is interleaved (i.e. RAID-0) or if it's RAID-1 (actual mirror). But you need to know what you're looking at / looking for. Hooking it up to a Windows-based PC is a bad idea, since Windows is crazy and enjoys trying to do things to the underlying disk the instant it shows up on the bus (*IX OSes don't generally do this).
And that's assume the drive even spins up -- when power-related problems occur (especially recurring brown and black-outs), the most likely failure point is the power-related circuitry. If the drive doesn't spin up, there are ways to solve that, but they're tricky and somewhat dangerous.
I can do the forensics/analysis for you if you send me the drive, but I can't make any promises that I can restore any of the data. I wouldn't charge you for the time, only any of the parts needed (i.e. replacement bits if the power circuitry turns out to be blown, and a spare/new hard disk which would have all of your data on it). I'd send you back 2 drives (the old/original and the new), assuming I was successful. Let me know.
Footnote question: why is this device not hooked up to a UPS? >:/ My advice is to hook it up to one ASAP. I can personally recommend the CyberPower CP850PFCLCD, although higher-capacity and lower-capacity models will work just as well (and will cost more/less respectively). These work wonderfully for power conditioning, emit an actual sine-wave signal rather than stepped/emulated, and (of course) will run off battery power when the power goes out. All I use mine for is avoiding the exact situation you just experienced with your HP product -- power outages in my town tend to be very short blips (under 1 second), and often repeat themselves at very short intervals (0.5 seconds), which is extremely rough on computer hardware.--
Making life hard for others since 1977.
I speak for myself and not my employer/affiliates of my employer.