Rev B (see Notes)
After blaming Startup Disk Creator (in the previous post) for its apparent inability to create an AMD 64-bit installation of Ubuntu on a PNY flash drive (although I didn't mention the drive name), it occurred to me the failure, which followed success with another brand of drive (Verbatim Store 'n Go), might not be due to some change in Startup Disk Creator, but due to the change in the brand of the drive, as strange as this seemed. So, I tried installing the 12.04 AMD 64-bit version on another 2GB Verbatim S&G drive, and it worked without any hitch whatsoever. Later, I tried it with a Sandisk Cruzer, and had no problems at all.
I'm not the only one who's noticed this problem with using PNY's for this purpose - go here and search for PNY. One possibility is that Startup Disk Creator might decide to insert defects in the AMD 64-bit installation after realizing that the target drive is a PNY. (Note that the brand is displayed in the Device section of SDC's GUI when you insert the drive.) Although this might seem paranoid, it's the only way I can explain why one of the most popular flash drive brands behaves differently than it does when using Universal USB Installer from PenDriveLinux.com. (It might not be the only one that has this problem, but I'm not about to go out and buy a sample of each variety of flash drive, and try to install the various types of ISO's on each one.)
Notes
Rev B: Added Sandisk Cruzer to the list of drives with which I succeeded in installing the AMD 64-bit version of 12.04, revised text for clarity, and deleted extraneous statements.
