What is the data structure of Linux? Is it FAT? Is it NTFS? Is it something else?

You mean the file system structure?

You've got choices - ext2, ext3, ext3, XFS, etc.
Those you mentioned are Windows file systems, so no - Linux does not use those.
You "can" mount those types of file systems on a Linux system by adding some extensions - but you won't "run" on them.

My question is based on the fact that I am trying to figure out why my external drive (in a case) hooked up by USB to my laptop is causing a laptop freeze and is almost unreadable. Both Hdd's (internal and external, as peripheral) are the same Linux: Xandros 3.0.3 Business. Regardless of whether Xandros is discontinued, or out of use, both the operational hdd and the peripheral are on the exact same o/s. Both the operational (internal) hdd previously held winxp (an NTFS file structure) and the external hdd held winxp, so both are newly installed. If Xandros did not re-format or did re-format, it's not an issue. The issue is why wouldn't the USB external be readable? And why does it cause a freeze? It worked perfectly on WinXP (which I loathe). So this boils down to 1 single question: What is the Linux data structure? Are they different? is Xandros different from, say Ubuntu? By the way, the same story hold true on the desktop. I also find that this same USB external is unrecognized in WinXP, which places Xandros one leg up if the external USB is at least recognized. The same USB external is recognized in Win98, but again unreadable due to slow/freezing. So if it can be surmised that both Xandros(Linux)and W98 are FAT why are they not fluid and readable? Anyone know the Data Structure of Linux? Xandros? Ubuntu?

But that question has no meaning, there is no such thing (linux data structure). The issue is about your usb device, what's happened, why does it cause a freeze (yes, that is fine, a really good question).
Xandros 3.0.3 probably unable to handle the filesystem on that usb device (or you can say the data structure on that usb is unknown), but you did not tell us anything about that so we can only guess.
If it worked on WinXP you can check the type of that filesystem on WinXP. Anyway, we need more information. What do you mean by "the same story hold true on the desktop"?

Do different USB connectors/cords handle only specific types of data? I assumed that a USB connection was a universal connection and transferred different data types universally. Not so?

usb is more or less universal, yes. The filesystem types are handled by the operating system - if they were capable to do that.

I think you have a fundamental misunderstanding of what a filesystem is. Xandros is not the filesystem, Ubuntu is not the filesystem. The OS does not dictate the filesystem, it does not control the filesystem. The filesystem is whatever you set it up to be when you installed Xandros. There are many options, you picked what you wanted, or you left it at the default. Run "df -T" if you want to see what it is.

Xandros can support many different filesystems, even more with optional add-on packages, same with Ubuntu. You can run Xandros or Ubuntu on ext2, ext3, ext4, btrfs, reiserfs, xfs, etc. You can also read/write to ntfs, hfs, fat, jffs, and more with the necessary packages installed.

If you have a problem with your external USB drive, then let's explore that, but get this whole "what is the data structure of Linux" out of your head because it's just confusing you. There is no data structure of Linux. Linux has no data structure. Linux can use or interface with many different file systems, just like Windows or OSX can.

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Then it remains to be determined why these (I counted four) new USB cord connectors failed to operate (show the hdd data) in fluid fashion. What data type is a USB cord capable of transferring? What is the Data Structure of Linux?

Is this a self powered USB external drive or does it have it's own power supply?
It has been reported that some systems cannot provide sufficient power through the USB port to adequately supply self powered external drives.

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And again: there is no Data Structure of Linux. USB cord is capable to transfer any kind of data.
If you need further help please understand we do not know the answer of "What is the Data Structure of Linux". And also please give us more details, and please answer.

Tell us more about the HDD you're using, and what "show the hdd data in a fluid fashion" means.

Digital data, bits. It's up to the host and device to agree on how these bits should be interpreted on either end. USB is out of the picture at that point, its only job is to get the bits from one side to the other in a timely manner without corruption.

As mentioned many times before, this question makes no sense. What kind of answer are you even looking for here? Let me offer a followup question - what is the data structure of Windows?

Thank you suicidaleggroll ! That was an excellent answer. I must learn about these "ext2, ext3, ext4, btrfs, reiserfs, xfs, etc." Where can I read about this? Patience please... I am a newbie! And yes, the USB must be the problem. But how to address this!

you can try the wiki for example: https://en.wikipedia.org/wiki/Ext4

The external hdd does not have a power supply. It just sits at the end of the connector. The laptop is a Dellc840 with no other power issues. The cords connects well (I researched the connector design flaws in the past and bought the right design) and the right cords.

does this device work properly in another pc/usb slot?