In August, I upgraded the hard disk in my MacBook to a roomy, 320-gigabyte Western Digital drive, effectively doubling its capacity. There was nothing wrong with the 160-GB drive that came out of it, and I've been pondering what to do with it.

I decided that I wanted to use it as a separate backup drive specifically for virtual machines. VMs take up a lot of room, and I've got quite a few of them - Windows Vista and XP setups from both Parallels Desktop 3.0 and VMware Fusion 2.0 on both my MacBook and iMac. Because they're large, I don't include them in my Time Machine backups. Every time a file changes in one of the VMs, they'd be rearchived on my Time Machine drives, which would take up a huge amount of space.

To turn the 2.5-inch notebook drive into a backup drive, I planned on buying an external enclosure. In Friday's Fry's insert in the Mighty Houston Chronicle, I spotted just what I was looking for - a Sabrent 2.5-inch Serial ATA enclosure for a paltry $9.99. I hustled over to the Fry's at Interstate 45 and West Road and grabbed one.

Unfortunately, once I got it home, I discovered that the Western Digital drive didn't quite fit. It was a bit too wide - or maybe the case was a bit too small. When I pushed it all the way in, the drive popped the end cap off. If I put the cap on first and slid the drive in, its flanges wouldn't fit inside the black aluminum case.

In addition, I noticed the case had a bad scratch on the side. Had this been opened before and returned? Fry's usually puts a restocking ticket on such items and there wasn't one here. Also, the box and enclosed materials didn't look like they'd been touched. I chalked it up to cheap design and went back to Fry's to get a different one.

This time, I picked up an AirLink101 AEN-U25SA. It was the same price as the Sabrent, which gave me pause. But I know several people who are using some of AirLink101's inexpensive wireless networking equipment who are happy with the brand, so I bit the bullet.

Installation went smoothly this time. There was plenty of room for the drive in the case, which was slightly wider and thicker than the Sabrent model. The front end cap includes a small circuit board and connector that plugs into the drive. I popped it into place and screwed it down with the supplied screwdriver. The drive isn't firmly fixed inside - it rattles a bit if you jostle the case. I wouldn't want to use this as a portable drive, but as a backup drive that stays on my desk, it's fine.

The AirLink101 comes with a USB 2.0 cable that allows for two connections to the host computer. This ensures the drive gets enough power, but I found it would work using just one. Still, when I transferred my 60-GB virtual machines files to it - which took about an hour per VM -- I had both connections plugged in, just to be safe.

The enclosure works with both Windows and Mac OS X systems, and requires no special drivers - just plug it in and start copying files.

While I probably could have paid a little more - 2.5-inch drive enclosures start around $10, with most costing about twice that - I probably could have one that held the drive a bit more securely. But this one will do what I need it to, so I'm happy.