Why does the advertised and available hard drive space not match?

November 13, 2013 7:57 AM

I ordered a Buffalo Drivestation Quad 4Tb external hard drive. It shows up as exactly 3Tb capacity. Am I missing something, or did they ship the wrong item in the right box?
posted by roofus to Computers & Internet (7 answers total)

Best answer: It's probably set up as RAID5. Short version: some of the drive space is used to protect against data loss.
posted by Sternmeyer at 8:03 AM on November 13, 2013

Hard disk manufacturers measure capacity in (decimal) terabytes, where 1 TB = 10¹² bytes. Everyone else in computing (including your computer's file manager) measures in (binary) tebibytes, where 1 TiB = 2⁴⁰ bytes. These are significantly different. 4.000 TB = 3.638 TiB.

So you should expect to see only "3.6 TB" listed as the total size of your drive. I don't know where the other 0.6 TiB went, though. I don't know any common file system with a 3 TB volume limit. Where do you see the 3.0 TB listed? How is the drive formatted? Is it using RAID? Can you open it in a drive/partition editor and see the arrangement of partitions and free space?
posted by mbrubeck at 8:05 AM on November 13, 2013

Seconding RAID5. Look at the "Five Operating Modes" on the bottom of the first page of the datasheet. The diagram is working off of a total size of 1TB, so you can see RAID5 Parity Mode gets you 75% of the drive as usable space. Presumably you could change the operating mode if you do in fact want 4TB of usable space, but remember that as data is stored across multiple drives with RAID, your failure probability is that many times greater. RAID 5 accounts for that, so if you don't use it, be sure to back your data up!
posted by Nonsteroidal Anti-Inflammatory Drug at 8:06 AM on November 13, 2013

Response by poster: I thought from the pattern of blinking lights on the front (perhaps wrongly) the drive was set for RAID1 out the box. It makes sense now that it might be in RAID5. Is that how I should leave it, assuming I do want redundancy?

In Disk Utility the Disk Description is "BUFFALO HD-QLU3/R5 1 Media" and the Total Capacity is "3TB (3,000,001,536,000 Bytes)". In the RAID "tab" in Disk Utility, there are no RAID sets listed. In the "Information" window, it suggests "Disk Number : 3", which made me wonder if one of the 1Tb drives had already failed.
posted by roofus at 8:39 AM on November 13, 2013

Best answer: Disk utility shows windows view of the disks and raid configuration. The drive station is most likely doing its own raid configuration internally and telling windows the end result not the internals.
You'll need to use buffalos raid utility to administer it not windows disk utility.
If you are in raid 5 one disk is used for parity, so you can only store data striped across the others. In case one drive fails you can use it degraded but still have all your data. If two drives fail you lose all the data.
posted by TheAdamist at 9:03 AM on November 13, 2013

Best answer: Sounds like you're on Mac, from the mention of Disk Utility? Either way, you'll only be seeing whatever volumes the device is configured to present to the host (normally one large volume), not the underlying configuration of the actual drives. As TheAdamist mentions, you'll have to make any changes to the RAID configuration through Buffalo's software, not Disk Utility. I don't have Disk Utility in front of me to check, but the "Disk Number: 3" may indicate either that it's the third disk seen by the system, or that the unit is presenting the single volume as LUN (logical unit number) 3 on the device. It should not be a concern, regardless.

It does look like you're set up for RAID-5 right now, which uses one disk's worth of space for parity information (but not one physical disk -- the parity is striped across all 4 disks along with the data). Your choices are:

1) Stick with RAID-5: You'll have 3 TB usable, but can only survive the loss of one disk. A second failure will destroy your data. Writes will be slower than RAID-1 or RAID-10, since any write will involve reading from all 4 disks in order to recalculate parity data.

2) Reconfigure for RAID-1 or RAID-10: You'll have 2 TB usable, but can survive the loss of one disk, and possibly a second. You'll have two mirrored pairs of disks (call them 1-2 and 3-4). If disk 1 fails, then you can survive a failure of either disk 3 or 4, but not of disk 2. So you've got a theoretical 2/3 chance of surviving a second disk failure, which makes it more reliable than RAID-5. You'll also get faster write performance, and potentially faster performance overall if you go with RAID-10 (which stripes data across the two mirrored pairs, with no loss of reliability).

So, it depends whether you're OK with RAID-5, or want to give up another 25% of the physical capacity in exchange for greater resilience and faster write performance (which depends entirely on what you're planning to use it for, but is unlikely to be an issue).
posted by McCoy Pauley at 9:23 AM on November 13, 2013 [1 favorite]

If capacity is more important than redundancy, you could set it up for RAID-0. This will give you your full capacity and the fastest possible performance, but the failure of any one drive will take down the whole system. So if you have a four-drive system, your failure rate will be 4x as high as it would be if you were on a single drive. RAID-5 will give you much better reliability than even a single-drive system (you can survive the loss of any one drive, giving you time to swap out the whole array [because if one drive fails there's a good chance others are soon to fail too] before you lose your data) at the cost of one drive's worth of capacity, a good compromise if you ask me. RAID-1 or -10 will give you even greater reliability, at a cost of 50% of your capacity.

If I were you I would probably stick with RAID-5. RAID-0 on a four drive system would not be worth the risk to me. If you absolutely need the full capacity you could either run RAID-0 with regular backups, or if your storage unit allows it you could have it register as four separate drives, so that the loss of a drive would only lose you the data on that drive – though that would be less elegant than having everything in a single filesystem.
posted by Scientist at 12:46 PM on November 13, 2013