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|Removable disk drives and cartridges|
|Click here for a comparison of removable disk drives and cartridges (PDF).|
While each removable disk vendor uses connectors to interface with the disk drive bay, the biggest differentiator is how they insert into the disk drive bay. RDX removable disk drive cartridges use a card-edge connector that produces friction when inserted into an RDX disk drive bay. This results in connector wear over time. Imation and Spectra Logic Corp. chose cartridge constructions that are more resistant to wear over time, according to the vendors. By using a pogo pin--a spring-loaded mechanism in their respective Ulysses and RAID eXchangeable TeraPack (RXT) Media--the removable disk cartridges are less susceptible to wear when inserted into the drive bay.
Users may also find that the actual number of times they can insert removable disk cartridges will run much higher than the guaranteed minimums to which they're rated. "Our life testing has demonstrated reliability for the RDX disk drive to about 10 times the minimum guaranteed specification of 5,000 insertions," says Steve Georgis, president and chief executive officer at ProStor Systems Inc., "and, in use, the RDX disk drives' specifications are 550,000 hours mean time between failure [MTBF]." ProStor's removable disk drive design is used by Dell and Tandberg Data.
Of course, the two biggest risks are data loss and disk drive failure. Data loss is a naturally occurring phenomenon on a disk drive that happens as the drive ages. Most disk drive manufacturers only rate the data as reliable for up to one year after the disk drive is powered off. While most disk drives include error-correction code in their firmware to deal with this eventuality, this firmware works only when the disk cartridge is powered on.
Most disk cartridges lack a mechanism to protect against data loss when the disk cartridge is powered off, but some, like Pro-Stor's RDX cartridge, will eventually include adaptive archival technology. This technology will act like RAID and spread the data across the disk. In the event data loss occurs in certain sectors of the disk while the disk cartridge sits idle, the removable disk drive recognizes that data loss has occurred and can recover data on the disk cartridge when it's brought back online. Because there's only one drive in each RDX cartridge, if a drive fails, all data is lost. Only Spectra Logic's RXT Media can recover from a catastrophic disk failure because RXT Media consists of many disks with factory-configured RAID 0, 1 or 5.
This was first published in March 2007