USB Flash Drives
Since giving away my iPod Shuffle to my wife a few weeks ago, I have had need of a portable storage device. Never before, but now I do. Go figure.
I have a firewire hard drive but it was either in use or largely unavailable and, even if available, I have to screw around with firewire cabling to make hook it up and then switch machines. A Shuffle would work, but iTunes is always going to fire up and ask to take over the device (if the machine doesn’t already own it).
Today, I finally got completely fed up with the situation after about the fourth time I needed to move a handful of files between two machines that were otherwise isolated.
So, I headed down to the on-campus Apple store to pick up a flash drive only to discover that they were out of stock. Now, that wouldn’t be that big of deal if I hadn’t been in the store earlier in the day and saw three 512MB units in stock at that time. So, apparently, I’m not the only person needing this kind of convenience.
On the way home, I stopped at Office Max to pick up a random cable. They had the Lexar 1 GB JumpDrive Secure on sale for $85 with an on the spot discount of $10 and a mail-in rebate for $15 — bringing the price to $60 (yup– math sucked the first time around). Not bad for a 1GB USB 2.0 (the high-speed 2.0) drive.
I’m ignoring the “secure” feature. Instead, I created a sparse encrypted disk image on the device that is the same size as the device. Sparse disk images start out minimally size and will automatically grow, as needed, up to the size of the media they reside upon.
You can create the disk image from within Disk Utility or the command line. The command line will look like (this is a single command):
hdiutil create -size 1g -fs 'Journaled HFS+' -type SPARSE \
-encryption -volname Name-Of-Volume /Volumes/FlashDrive/Name-Of-Volume
I would recommend not placing the disk image password in your keychain. The end result is a convenient, cable-less, tiny portable storage device with unencrypted and encrypted storage areas that automatically grow/shrink as your needs change. No third party drivers or software needed.
Update #1: As Eric noted, the resulting image will start out at 34MB for an empty image. That appears to be overhead related to creating the disk image. As the size of the image drops, the amount of overhead drops along with it — to a point. To answer another question of Eric’s, hdiutil compact image will recover disk space no longer used by the sparse image and return it to the underlying file system.
Update #2: Bob Ippolito suggests that journaling an encrypted disk image is largely pointless. He claims that it will increase the potential for corruption and the additional duplication of information necessary to journal the filesystem will necessarily imply less security.
Honestly, I don’t really have enough information to form an opinion in regards to the likelihood of corruption with or without journaling. As for security, it is true that any repetition of data within an encrypted stream decreases security. In this case, the decrease in security is likely extremely miniscule.
But there is another reason to turn off journaling and it gets back to Eric’s point about the overhead. Turning off journaling reduces the “out of the box” size of a 1GB sparse, encrypted, disk image from 34M to 26M. A significant savings.
Some more data:
| FS Type | 1g empty image size |
|---|---|
| HFS+ Journaled | 34M |
| HFS+ | 26M |
| HFS | 4M |
| MS-DOS | 3M |
| UFS | 89M |
Interesting and not terribly surprising. For now, I think I’ll stick with HFS+ as I have plenty of space. hdiutil can always be used to convert the image from one format to another later.