Back in 2001, the iPod was the first widely sold product to use one of these little hard disks. With a diameter of just under 2 inches, it’s smaller than the hard drives used in most laptops, yet it can hold up to 60 gigabytes of data, or around 15,000 songs. The midsize iPod Mini uses an even smaller version, albeit with decreased capacity. It’s just an inch in diameter but holds up to 6 gigabytes of data, enough for around 1,500 songs. (The lowend iPod Shuffle doesn’t use a hard disk, and it holds relatively few songs in its memory chips.)

But now these little hard disks are migrating to other devices — including cellphones and personal digital assistants — made by other companies, giving these gadgets some of the iPod’s magical combination of diminutive size and expansive capacity. And since these devices can play music, along with the various other functions they perform, they could soon become a challenge to the iPod.

For example, two big cell phone makers, Samsung and Nokia, have designed music-playing phones with small internal hard disks that hold a few gigabytes of data. Samsung’s hard-disk model is sold so far only in Korea, but could make it to the U.S. by the end of this year. Nokia’s will be rolled out late this year, probably first in Europe.

By late 2006, I expect Americans to have numerous choices in hard-disk cell phones.

The first PDA with a hard disk to be offered in the U.S. came out in May. It’s the $499 PalmOne Life-Drive. PalmOne sells the LifeDrive — with its large color screen and 4GB hard disk — as a portable way to store and view or play back music, videos, photos and office documents. It also features the usual Palm calendar, contact and notes functions, and with its Wi-Fi and Bluetooth wireless networking, it can surf the Web and send and receive e-mail.

The LifeDrive is bulky — much larger and heavier than an iPod Mini with the same size hard disk. And it isn’t a great music player — it has touch-screen play, pause, fast forward and reverse rather than proper buttons, and it doesn’t come with headphones. But it could be the start of a trend for PDAs, since it should be possible to wedge a similar hard disk into a smaller device.

More likely, the bigger threat to the iPod will come from iPod-enabled cell phones. People have to carry their phones anyway, and some already offer music-control buttons and headphones. So if your phone can hold thousands of songs, why carry around a second, separate music player?

Of course, phonemakers still have to prove they can design music-playing phones that are as simple, elegant and cool as iPods. But that could happen.

Little hard disks could also revolutionize digital cameras, allowing photographers to store thousands of shots without lugging around a laptop, although I know of no camera to date that has a hard disk.

Another trend: Small hard disks will likely shrink even more. Already, there’s a company making one with a diameter smaller than an inch. But mini hard drives may also face a challenger of their own — high-capacity memory chips. The chips have tended to be costlier per unit of storage than the disks, but over time they could get competitive. Prices are dropping fast, and chipmakers are working on memory cards, like the one in your digital camera, that are capable of storing over 10 gigabytes.

For now, though, keep your eye on the little hard disk that powers the iPod. It may be powering lots of gadgets soon.

This week, my assistant Katie Boehret and I tested the first such high-capacity PDA, the $499 LifeDrive from palmOne — the company that, in its original incarnation as simply Palm, developed the first popular PDA, the Palm Pilot, back in 1996.

PDAs from Palm and other companies could already hold some of these multimedia and productivity files. But their storage systems, which used memory chips, were severely limited and functioned mainly as organizers for calendar and contact data, short notes and a limited amount of email. Those functions are increasingly migrating to smart cellphones, so palmOne is hoping to revive the PDA with the built-in hard disk and a new role: managing and viewing your important digital files when you’re away from your computer.

The $499 LifeDrive mobile manager from palmOne Inc. stores and displays photos, documents, music and videos.

At first glance the silver, rectangular LifeDrive looks like a thick, high-end organizer with an extra-large screen. But under the hood it has a four-gigabyte hard drive, which gives away the fact that there’s more to it — if the hefty price didn’t tip you off.

PalmOne wants you to look at the LifeDrive not as a bulky (6.8-ounce) handheld on steroids, but rather as a “mobile manager” — meaning that this thing will not only carry your data on one portable hard disk, but also allow you to view, edit and listen to that data in a comfortable, uncomplicated way. It also uses Wi-Fi to access the Internet for Web browsing and emailing.

Overall, we found this portable to be user-friendly and well-designed, if a bit large. We really liked how its generously sized 3.8-inch color screen can flip from portrait to landscape view with one simple click of a button on the LifeDrive’s side. This made Web pages more comfortable to read and videos easier to watch. A handy five-way navigation button at the base of the device lets you page through menus and lists of options with one hand.

But the LifeDrive disappointed us on one important count: poor battery life. When we used the device with its Wi-Fi turned on, it ran out of battery after just two hours and 18 minutes. We also tested battery life by playing music files in a repeating loop with the screen on; the LifeDrive did a little better but still lasted only three hours. More disturbingly, this last test crashed the LifeDrive three times.

In addition to Wi-Fi, the LifeDrive has built-in Bluetooth — a short-range wireless technology that connects the device to other Bluetooth-equipped products like cellphones, printers and computers. After a few clicks, Katie had our device recognizing my Bluetooth-equipped Apple iMac G5 desktop computer.

For more information: www.palmone.com.

Transferring data onto the LifeDrive is simple and can be done in a couple of ways other than wirelessly using the Internet to load files. A handy SecureDigital (SD) card slot at the top of the device makes it easy to offload digital photos from a camera. Katie’s digital camera takes an SD card, so she was able to insert the card and dump photos onto her LifeDrive on the go, freeing up precious space on her camera. If your camera doesn’t take an SD card, palmOne’s $50 Digital Camera Adapter accessory attaches to the device and acts as a reader for other types of memory cards.

We also loaded our computer files onto the LifeDrive. Once it was connected to our computer, it acted as another drive. A smart program called the LifeDrive Manager popped up on our desktop and displayed five folders for loading Applications, Documents, Music, Photos and Videos. We easily dragged and dropped documents into the appropriate folder, and they were transferred to the LifeDrive.

Of course, you can also HotSync your LifeDrive like a traditional palmOne PDA once it’s connected to your computer. That process synchronizes the address book, calendar and other organizer features with Microsoft Outlook or other organizer software on your computer.

The LifeDrive comes loaded with programs that let you use your digital files. DataViz Documents To Go 7.0 makes it simple to view and edit Microsoft Office files created by PowerPoint, Word and Excel. We opened and edited Word and Excel documents without issue. Special palmOne programs such as Pocket Tunes and Blazer handle playing music and Web browsing, respectively, on the small device. And another program plays entire movies — we watched a little bit of “The Lion King” in landscape view and were impressed with the results.

We listened to music and video soundtracks on the LifeDrive out loud through a speaker on the back of the device, which produced a pretty hefty sound considering the size of the thing. You can also listen by plugging headphones in, though a set is not included.

After setting up the LifeDrive’s email program to access my account, we easily sent attachments and text emails from the LifeDrive.

The palmOne LifeDrive is a pretty fair little gadget for offloading important digital data that you want to keep with you while traveling or on a day-to-day basis. It would be especially handy for digital camera users who want a good way to offload photos when they can’t get back to their computers to do so.

But it’s expensive, it’s bulky, and its poor battery life is really limiting, especially for people who will want to access their media and use the LifeDrive for Internet access over the course of a whole day. Until the company can improve battery life of the LifeDrive, we can’t wholeheartedly recommend it.

Even if palmOne fixes the battery issue, it’s not clear that the inclusion of the hard disk can revive the fading PDA category. Smart phones are also getting hard disks — the first prototypes of hard-disk phones have already been shown in Korea and Finland. And once such hard-disk phones become common, they should be able to do everything the LifeDrive does and more, albeit with a smaller screen. So why carry a second, bulky device?

With reporting by Katherine Boehret

Write to Walter S. Mossberg at mossberg@wsj.com