KenK wrote:

... looking for an xp external HD. No luck. All I've checked out are
not compatible, almost always Windows 7 or later.

Only if you give a gnat's fart about the software bundled with the USB
drive. Just use the external drive as a generic USB mass storage device
in Windows: plug it in, Windows will find the embedded driver or
interface definition file, and you're ready to go - just like when you
plug a USB flash drive into a USB port.

If you need backup software, and what comes bundled with the drive has a
Windows 7 minimum requirement, go get other backups software ... and for
free. Macrium Reflect Free and Easeus ToDo Backup Free will run on
Windows XP. Most software will require SP-3 but you didn't mention your
service pack level of Windows XP.

If the bundled software includes an accelerator program that claims to
speed up USB transfers, toss it. If you copy one or two small files,
the accelerator won't help (the difference in transfer time is
insignificant). If you copy a huge file, the accelerator won't help
(buffer too small). Only if you copy LOTS of small files might the
buffering afforded by the accelerator provide reduced transfer time.
Plus it is more software in the chain that makes the chain less
reliable: just one more thing to go wrong and you may not get any
support for it.

The hardware is compatible with Windows XP. It's the bundled software
that may not be, so don't bother with the bundled software. It's
cheapware added to bloat the feature set so marketing can make their
product appear more than just a simple USB HDD.

Any suggestions? Somewhere around 1 or 2 T and under $100.

Not for under $100 if you want to stick with a name brand (Seagate,
Western Digital, etc). More like up to $150 to get up to 2 TB since
that's the top end of your requested storage capacity. Newegg has some
non-name brand 1 TB USB HDDs for under $100 (e.g., Fantom) but I have no
experience with those resellers that buy the drives in bulk, put them in
a case, and resell them under their name. They have some name brand USB
HDDs at 2TB for under $100 but they are refurbishe (aka used, inspected
visually, but rarely ever actually tested). Newegg is not always the
cheapest but they are reasonably priced and I've had good experience
with returns to them (make sure you buy from Newegg, not another seller
using Newegg as an e-store, just like how Amazon, Walmart, and others
sell e-store space to other sellers), plus they often have some very
decent sales.

https://preview.tinyurl.com/ydf7dn5b

That'll find some lower prices than Newegg except you are often buying
from unknown sources or small sellers with crappy support. Some stores
for a listing are good, some are not. If you've been buying computer
parts for awhile, you'll recognize the okay or good sellers. The above
search found some 2TB WD USB HDDs that Newegg's own search (with my
criteria) didn't find.

Windows XP only supports up to USB 2.0. If you get a USB 3.0 USB HDD,
it will only transfer at USB 2.0 speeds (and not even at the top rated
or theoretical limit but more like around 70% of the speed claim).
Because you are stuck with USB 2.0 with Windows XP, the USB 2.0 ports
can only provide 0.5 amperes which is likely not enough to power the USB
HDD, especially during spinup. Not only do you have a spinup surge load
when powering up the drive but quite often USB HDDs have firmware code
that make them go to sleep which means then spin down - so you might
want to ensure you do NOT get a "green" USB HDD because that might not
wake up fast enough during a backup job that has to pause for a while
compressing a huge file before sending to the USB HDD. Some USB drives
come with a Y adapter cable: plugs into 2 USB ports to get up to 1 A to
power the external drive and the logic PCB inside the enclosure. With
USB 2.0 and Windows XP, you should look for a USB HDD that has its own
power source (A/C adapter) or make sure the USB HDD will run reliably on
just 0.5 A (or 1 A with a Y adapter).