"could not obtain the target drive letter"
I realize this is too late to help you, but perhaps it will help someone else.
The problem isn't due to a hard drive failure or faulty RAM (though testing the hard drive with Hitachi Drive Fitness Test and testing the RAM with Memtest86+ are good things to do).
I finally figured out the cause & solution to the VAIO "Could not obtain the target drive letter" problem.
Background: my customer's VAIO laptop died with a bad hard disk drive. I did linux-based data recovery (using GNU ddrescue) on the hard disk drive, and managed to recover the recovery partition (about the first 6.2 GB of the drive), but most of the C: partition was unrecoverable. (Fortunately, the customer didn't need the data on it, she just needs the computer to work.)
So I imaged the recovered recovery partition and the first few hundred megabytes of the damaged C: partition onto the new hard disk drive. I then tried to use the recovery partition to restore a working Windows Vista installation, and had a horrible time of it. I kept running into the "Could not obtain the target drive letter" error message.
Sony's tech support was useless: the phone support person turned hostile and hung up on me when I asked whether I'd get my $40 back for the support incident if they were stumped. Their online chat person was friendly but had no clue what to do, though he told me that I could call 239-768-7691 to buy a recovery disc. But I finally figured it out.
Sony has a bug in their recovery code. Some of their code assumes that the recovery partition is the 2nd partition, and some assumes it's the first partition. It is really the 1st partition, so when you bang F10 (or maybe Alt-F10) and get to this screen...
Edit Boot Options
Edit Windows boot options for: VAIO Recovery Environment
[ /DETECTHAL /MININT /REDIRECT RDIMAGEOFFSET=8192 RDIMAGELENGTH=3161088 RDPATH=multi(0)disk(0)partition(1)\sources\boot.wim ]
...you need to edit the path to boot.wim, to reflect the fact that it's actually on the first partition. Just left-arrow-back and change "partition(1)" to "partition(0)".
Then when you get to the System Recovery Options screen, choose "Command Prompt."
Then do "chkdsk c: /f" to fix the C: partition.
Then "dir c:" and if there's no C:\Windows\ folder, then do "mkdir c:\Windows" and "mkdir c:\windows\system32" to make one (this step might not be necessary).
Then choose "VAIO Recovery Center" and the complete OS restore should finally work.
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