Since running FlexRAID, I'll assume you want to run snapshot RAID and not real-time. FlexRAID real-time is still in it's early stages and even the developer, Brahim, does not recommend relying on it.
I've been using FlexRAID since it's early beta days for years so my setup is 'manual' and I'm not familiar with some of the newer automated features, so maybe this is different with my setup? With snapshot, you really don't have an 'array' in the way you would think of a typically RAID array. It's just a bunch of disks which you define the FlexRAID software to look at to generate a parity off of. You can add / remove as many drives as you want, whenever your want, with whatever data on them. There is no service to stop or concern of loss of data or corruption if you remove a drive. All you have to do is run an 'Update' and your parity will scan all drives defined for the snapshot pool and generate your new parity. If you are adding a hard drive with no data, then you can run 'Fast RAID Expansion', but you can still run an update with the same result...it's just slower.
Most of us setup a scheduler (built into FlexRAID) to run snapshot updates for us. I set mine to run every morning. FlexRAID will cancel an update if it detects a hard drive is missing (this means your OS can't read/find the drive). If your drive is corrupting data, the snapshot will still run, and update the parity with the corrupted file (it won't corrupt your parity, it'll just restore the corrupted file). FlexRAID has a SMART monitoring service built-in that you can set a scheduler to check your hard drives and send you an email notification if a SMART status changes. This by no means guarantees you'll know when a drive is corrupting or about to fail, but it can help.