I have a iMac 27”, 5K with a 2TB Fusion Drive. I am running my Time Machine backup on an OWC RAID Enclosure with two 4TB hard drives. I am not using the hardware RAID that is part of the OWC enclosure, but the software RAID through the iMac. Every so often I get the error message that the drive has been improperly disconnected. The drive is connected directly to one of the USB 3.0 ports in the back.
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iMac 27” (2017) RAID Disconnects
- Thread starter cwatson69
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With software raid, if one drive fails you can easily replace the drive. With hardware raid, if one fails you’ve really got to watch what drive you get to replace it. Sometimes you can have the same size drive but a different brand and it won’t work.
Never heard of that being an issue, but fair enough. On the other hand though, hardware RAID is transparent to the software, whereas software RAID can break more easily, with software updates rendering your OS incompatible with RAID systems made with earlier systems or whatnot, and it's often unrecognisable by different systems, as well as impossible to boot from in a lot of cases if you'd want that. Oh and it eats CPU cycles to handle.
But sure, with your argument as well, it's a trade off, so one should pick whichever option one prefers. - Sorry for the off-topic aside. No clue why it randomly disconnects like that
If this is the OWC Elite Pro Dual, it has both USB 3 and Thunderbolt 2 connectors. It has switchable hardware RAID-0/1, so you can use either hardware or software RAID-0/1.
I've never had a problem with a Thunderbolt drive or array disconnecting. I have had several problems with USB drives disconnecting -- not frequently but every now and then.
If possible I recommend you use Thunderbolt not USB 3 on a RAID system. It's not about performance it's about reliability.
Another possibility is your RAID drive's AC power may be unreliable. If you are not using the OWC factory AC adapter that is easily possible. A non-OEM AC adapter may meet the voltage standard and the drive may spin up but under heavy seeking it can exceed the current limit and the drive will shut down, which shows as off line to macOS.
A related possibility is a cable to the AC adapter on either AC or DC side could be loose and not reliably connected.
Yet another possibility is a drive in the RAID unit may be marginal. Ideally this should be reflected back to macOS as a S.M.A.R.T error or retry count exceeded. However that doesn't always happen, and can depend on the hard drive, internal controller (if used) or the RAID software.
I usually use SoftRAID on all my RAID units and it's very good about propagating hardware errors back to macOS. I use that for the same reasons you state -- avoiding proprietary lock-in based on a low-level hardware RAID format.
No, it just happened now while I was using it.
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Mine doesn't have a Thunderbolt 2 connector. It is a "OWC Mercury Elite Pro Dual RAID USB 3.1 / eSATA Enclosure Kit"
I've only had this unit since Dec. 2017.
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Have you tried another USB cable?
Some times it happens to have problems. I had disconnect problems with a Samsung T3 SSD, replacing its cable fixed it.
Where possible it's better to use Thunderbolt for RAID arrays. It's not a matter of performance but one of reliability and ease of configuration, e.g, you can string Thunderbolt drives together.
That said, we don't know for sure your issue is caused by USB. I have many RAID arrays, mostly Thunderbolt but a few USB 3 models. In general they both work OK, but I've had more spontaneous disconnects from the USB models (though not a large number).
As filmak stated, check your USB cable, make sure it's the original one and it's not damaged or loose. Try a different USB port on the iMac. Don't use a USB cable of different design or length.
Another possible cause is an intermittent or emerging hard drive failure. I had a two RAID-0 arrays fail last week, one an 8TB G-RAID and the other a 16TB OWC Elite Pro Dual (Thunderbolt version). In both cases a single drive in the RAID-0 array failed. The G-RAID was only a few months old, so this can happen on new drives. The OWC was about 18 months old. Neither of manifested as a disconnect. The G-RAID was under SoftRAID control, which reported an emerging problem due to high retry counts. The OWC drive was using hardware RAID and it just failed, which crashed macOS, even though it was only a Time Machine drive. In both cases I replaced both drives in each RAID on the principle that if one HDD fails the other is more likely to fail, sometimes called "sibling mortality" syndrome.
It's probably a good idea to reformat the drive and run a full surface validation test on it. There are several utilities which can do this; I've used
Scannerz: http://scsc-online.com/Scannerz.html
DiskTestR: https://diglloydtools.com/disktester.html
SoftRAID: https://www.softraid.com
Micromat has a S.M.A.R.T monitoring utility called DriveScope: https://www.micromat.com/products/drive-scope
As casperes1996 said, it is conceivable it's caused by an AppleRAID software issue. I think that's somewhat unlikely -- I've used AppleRAID a lot on two-drive RAID-0 arrays without problems. However it's possible. From a troubleshooting standpoint you could temporarily try changing that to hardware RAID since the Elite Pro Dual supports it. It would have to be reformatted for this.
If you plan on using software RAID, I suggest SoftRAID, which is an excellent, very reliable product. For RAID 0/1 you don't need it but it's constantly maintained and improved (inc'l performance), whereas I don't think AppleRAID has been improved for a long time. https://www.softraid.com
Yes it's paying money for functionality you already have but if you ever want higher RAID forms like RAID-5 it supports that. I've done lots of performance testing of SoftRAID in RAID-5 vs the hardware RAID on a Promise Pegasus R4, and SoftRAID was faster in execution and much faster in the rebuild or formatting phase. The host CPU overhead was minimal.
The original cable is only about 18" long, so I had put an extension on the end of it.
I've now moved it, got rid of the USB extension cord, and now the original cable is plugged directly in the back of the computer.
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As I replied to film above, I've moved the drive and have gotten rid of the USB extension cable that I was using.
Yes, from a troubleshooting standpoint I could try all of your suggestions, but I decided to go ahead and download the trial version of the softraid software. (I downloaded the full version and not the lite version.) I've installed it and everything is going fine so far. We'll see how it does this coming week.
Hopefully the shorter cable will help. Just to be clear, when using SoftRAID you'll should switch your OWC drive back to JBOD mode (IOW don't use the internal RAID controller). That of course requires reformatting it with SoftRAID which means any data should be backed up. However it's a Time Machine drive so maybe you don't need to back that up.
I only mentioned SoftRAID for completeness, not advocating it as a clear-cut solution for your problem. AppleRAID functionality was removed from Disk Utility in El Capitan but it has since been restored.
After using SoftRAID for years on over 200 TB of drive arrays, I trust them. However AppleRAID should work OK, as should the built-in OWC hardware RAID controller.
SoftRaid is way more reliable than AppleRaid.
I had some problems with AppleRaid in the past, I do not know if its new restored version is improved or not.
And yes, Thunderbolt is more reliable too.
I had some problems with AppleRaid in the past, I do not know if its new restored version is improved or not.
And yes, Thunderbolt is more reliable too.
Good to know the cause of the problems.
SoftRaid is indeed better and with more options. I hope it will stay without problems with your setup from now on.
SoftRaid is indeed better and with more options. I hope it will stay without problems with your setup from now on.
In case of anyone who uses USB3.0/3.1 port in Mac OS has dropped out the issue. Please run this utility it may help
https://www.macupdate.com/app/mac/31158/keep-drive-spinning
https://www.macupdate.com/app/mac/31158/keep-drive-spinning