One disk crashed, then another shortly after.

Well powering down, connecting the failing disk up to a PC and cloning the failing disk using dd_rescue (e.g. using a Linux Live CD, there are instructions on the forum for this) would be one option.

Note if the disk is failing dd_rescue may not be able to successfully clone all of your data but it should help you to recover most of it.

What you recover you should compare with what you have in your backup (though it sounds like you may not have made one) as some files may be corrupt. The copy in a backup from before the disks in the NAS had issues may be more reliable.

You may also wish to make an Online Submission (see the link in my sig) and see what support suggests.

Your drive's heads have parked themselves to death. This is very common with Green drives, and I have successfully restored data from an array with four Green drives in RAID5 (in a Qnap device). You can use raid reconstruction tools and likely get most or all data back.

Note that the array doesn't need to be used to die -- on th contrary, heavy usage might stop the drives from killing themselves, as they cannot park their heads as often.

I have been asked to provide details on my recovery procedure... unfortunately, since you used X-Raid 2 (which is LVM), the software I used won't work for you as it doesn't support LVM. :( Bummer. Will provide details anyway.

Basically, I finally used the R-Studio software from R-Tools Technology after several failed attempts assembling the array in Linux, trying to resync it, using Testdisk, etc. I attached three of the hard drives with SATA-to-USB adapters (do note, SATA will be MUCH faster but at the time I did not have access to a recovery workstation with enough SATA ports) and instructed the software to reconstruct the array with one missing disk -- it simulates the missing disk and recalculates data from the remaining disks on-the-fly, this is useful if one of your disks hangs for example. You can input different RAID parameters, then just choose your geometry and preview the disks. It supports EXTx out-of-box, and to verify that your settings are correct, try extracting a fairly large file and have a look at it, with a video clip, image or a zip you will immediately notice if anything is broken.

Try finding software that will work with LVM. Most recovery software have a free trial mode. Work from images/mirrors of the original disks if at all possible.

I found using Linux to recreate the array pretty useless, as the array would drop immediately on any error. Only software that actually ignores if the array is broken or not will work. Then again, it might be possible to do it in some way I didn't found out. :)