Start with your original MBR configuration and use a Windows recovery tool to try to restore its boot loader. With any luck, that will get it to work. Do not switch from MBR to GPT on the new disk.
You should first understand the difference between the GUID Partition Table (GPT) and the Master Boot Record (MBR) partitioning systems. The former is the native partitioning system for Intel-based Macs and for newer PCs. The latter was used on most PCs prior to the release of Windows 8. GPT is associated with booting in EFI mode (as OS X boots), whereas MBR is most commonly used to boot BIOS-mode OSes (including Windows XP). Importantly, part of GPT is a protective MBR, which is basically an MBR data structure with a single type-0xEE partition that covers the entire disk (or 2 TiB of it, on disks larger than this). If the protective MBR is not present, or if it doesn't fit this description, then the disk is not, technically speaking, a valid GPT disk.
Apple uses an ugly and dangerous mixture of GPT and MBR, known as a hybrid MBR, in some of its dual-boot configurations. Your original disk was probably configured in this way. In a hybrid MBR, up to three of the "real" GPT partitions are duplicated in the MBR, and the MBR's protective type-0xEE partition is shrunk accordingly. This means that a hybrid MBR is technically not a valid GPT disk; but OS X and Linux both treat it as if it were a GPT disk. Windows, OTOH, treats disks with hybrid MBRs as MBR disks. This is useful when dual-booting OS X in EFI mode and Windows in BIOS mode, but it's a dangerous hack because so many things can go wrong with it. (I won't go into details because they'd be a digression.) The "syncing" of GPT and MBR to which you refer is creating a hybrid MBR. This isn't really relevant in your case (at least, as originally stated).
You should also understand, at least in outline, how Windows XP boots: BIOS boot loader code is stored to the MBR (the first sector of the disk). When the computer boots, the BIOS loads this code and executes it. This code then reads additional boot code from the Partition Boot Record (PBR; the first sector of the Windows XP partition). The PBR code continues the boot process by reading additional files, eventually launching the Windows kernel, etc. The key point here is that neither the MBR nor the PBR is a file; they're lower-level data structures that define partitions (the MBR) or filesystems (the PBR) and contain boot loader code. Given the way you copied the Windows installation, the PBR was copied, but the MBR was not. Another point is that some versions of the Windows boot loader require that the Windows partition have its "boot flag" (aka "active flag") set.
This interacts with Macs specifically in that Apple uses the partition table type (pure GPT vs. either MBR or hybrid MBR) as a key to decide whether to activate the Compatibility Support Module (CSM), which is a sort of BIOS emulator that enables Macs to launch BIOS-mode OSes like Windows XP. If the disk has an MBR or a hybrid MBR, the Mac activates its CSM and may launch a BIOS-mode OS; but if the disk is a pure legal GPT disk, the Mac leaves its CSM inactive and so will not be able to boot Windows XP. (BIOS-bootable optical discs can also activate the CSM.)
There can be other boot problems, too. For instance, the boot loader might refer to files that don't exist or have hard-coded sector values that are no longer valid after you've copied your files. Windows is pretty notorious for this sort of thing, and you must generally use a Windows emergency disk to recover from such problems. In fact, that's what I recommend you do. Unfortunately, I don't know enough about Windows recovery procedures to give you explicit step-by-step instructions; but if you search online, there are dozens of sites that describe how to do this. You'll need a Windows XP recovery disk that you can boot and use to fix your copied Windows installation.
Speaking more broadly, if the external disk houses Windows XP and does not include a bootable OS X installation, I recommend you use a straight-up MBR configuration on it. Windows XP will not boot from a GPT disk unless it uses a hybrid MBR, and such a configuration would add complexity and make it more likely that you'll run into future problems without providing any benefits.
Note that Spiff's claim that Macs can't boot from MBR disks is flat-out wrong. I've done it myself, in both BIOS mode and in EFI mode. Macs can be flaky, though. This is especially true when booting from external disks in BIOS/CSM/legacy mode. Thus, it's conceivable that you'll never get this working from your external disk. OTOH, it does work on many systems, so maybe you'll manage the task.