Not So Fast, Flash Drives

Q. I make double backups, one on an external hard disk and a second on a high-capacity thumb drive. As the thumb drive has no moving parts, why does it take four to five times as long to write to it than to the hard drive? A. Thumb drives use solid-state flash memory to copy data back and forth with the computer. Compared to traditional hard drives, which are composed of motorized, magnetized platters spinning away, flash drives are smaller and more durable. But there are a couple of areas in which hard-drive technology still beats that of flash drives: the price per megabyte and the amount of time it takes to write (copy) data to the drive. For example, a hard drive may write 55 megabytes of data a second, whereas a high-capacity flash drive may only be able to manage about 20 megabytes a second. (Coding Horror is among the many sites that have done personal comparison tests and reported results; check out snipurl.com/d58ql.) Most flash drives list the write speed in the product’s technical specifications, which can be helpful when shopping, but doesn’t do you much good when you have already bought one. Other factors may affect the flash drive’s performance, like the condition of computer’s U.S.B. port and the quality of the flash drive itself. Although it erases everything on the drive, reformatting may help a U.S.B. stick that has gotten progressively slower or erratic. You can do this either with software from the drive manufacturer or from the operating system itself. The Woopid site has video tutorials for the current versions of Windows (snipurl.com/d5aoj) or the Mac OS X (snipurl.com/d5aq2). How to Clear Download Clutter Q. It drives me nuts when I download a program, or upgrade an old one, and it arrives with assorted junk. What am I supposed to do with the .dmg file that opens the program? Junk it, or save it for future use? A. Disk images (.dmg files) act like a virtual version of a hard disk or flash drive, which is why they pop up like portable drive icons on the Mac’s desktop when you open them. The file or program you downloaded is stored on this virtual disk. To install the file on your Mac, you usually just have to double-click the installer files or drag the files to your Mac’s hard drive. The .dmg file is basically just the container for the program or file you were downloading to the Mac. Tossing it in the Trash after you’ve installed the software is fine. But for backup purposes, you may want to make a copy of the file that was on the disk image. You could, for example, round up all the files contained on those downloaded disk images and burn them all to a CD for safekeeping before you trash the .dmg files. TIP OF THE WEEK: Tired of squinting at tiny type on the screen all day? In Windows Vista, you can bump up the font size — right-click on the desktop, select Personalize and click on “Adjust font size.” Once the “DPI Scaling” window comes up (probably after a Vista security alert), select the larger “120 dpi” setting and click OK. Illustrated instructions are at snipurl.com/d5bl9. On a Mac OS X Leopard system, you can increase the size of type on desktop icons by going to the View menu to Show View Options; the text size controls are there. Mac users can zoom in on the screen by pressing the Option-Command-plus (+) keys; pressing Command-Option-minus (-) zooms out. Pressing the Command key and either the plus or minus keys increases the size of the text in certain Mac programs like Mail and Safari. J. D. BIERSDORFER Questions about computer-based technology may be sent to QandA @nytimes.com. This weekly column will address questions of general interest, but e-mail and letters cannot be answered individually.