I'm running Ubuntu Server 10.10, with software RAID5 and LVM running on top of that. I have four 2TB HDDs. I also have a separate boot drive with incremental full-system snapshots that run every four hours, and every day, week, month, and year. So even if the RAID array totally fails me, my data won't be lost, but it'll be difficult to re-setup my entire system.
I tested my software RAID5 the other day by pulling a HDD while the system was off, booting degraded, putting back the HDD, rebooting, and then rebuilding after telling mdadm to re-add the re-inserted drive. After 10 hours of rebuilding, it succeeded, and now the software RAID5 device is clean again!
Now, I want to do another test. Would you say it's dangerous to pull one of the HDDs while the machine is running and possibly reading/writing? I'd pull the SATA cable, not the power cable, so that I don't risk damaging the hard drive, but would I have data loss? I have very redundant backups on other HDDs so there's no risk for permanant data loss, but I don't want to have to reinstall and reconfigure my entire system.
Is it unsafe to test? What's the worst that can happen?
I'm the original poster. Because I made this question without an account, and because it was moved to a different StackExchange site, I lost ownership of the question.
So I did the test. I pulled the SATA cable of one of the four hard drives, and the system performed beautifully.
Here's a screenshot of the Webmin RAID interface:
And here's a screenshot of an email I received from my server:
So to answer my own question, it was safe to do so in my particular case.