Can I install 2 new disk at once?

You can put in 2 at once, the NAS will resync with one, then with the other.
Probably sensible to do one at a time though, if there is a problem it's slightly easier to troubleshoot without the presence of another disk.

popped in the 3rd disk and it said it basically what you said so I popped in the 4th disk.

Restriping 0% complete, Time to finish 253 hr 29 min, Speed 1065 KB/sec

over 10 days to restripe? Somebody tell me that's not correct.

This is exactly what I meant when I suggested doing one at a time.
What model readynas?
What firmware?
What model disks are you adding?
What disks were in the NAS?
What raid format?
Do you have a backup?

I agree one at a time is best, especially if something goes wrong. Though given where you are, just leave them both in.

The time estimates are not reliable - in some phases they turn out to be much too long, in others they are much too fast. They also depend on what else you are doing, since the resync is a background task.

A 3 TB drive takes about 12 hours to sync in my pro-6, so it shouldn't be longer than 24 hours per drive, and could well be less.

Got a 4200 V2
Most current Raidator
WD RE 2x 1tb adding 2x2tb
X-Raid 2
I do have a backup

Might be faster to factory reset the NAS with all the drives in then move the files back.. what do you think?

btw: The restriping is speeding up. I was transfering files in the background and forgot about it. It's taking 21 hours to restripe now.

It may be quicker, however you won't have access to your data ( at least not in the normal way) and you'll only have one copy of the data.

If the 1 TB drives are healthy, I'd just let it complete. It should beat the time estimates.

The first stage will horizontally expand to the first 2 TB drive, giving you a 2 TB total volume.
The second stage will horizontally expand to the second 2TB drive, giving you 3 TB total.
The last stage is a vertical expansion on the 2 TB drives, giving you 4 TB total.

You probably will need to reboot to the last stage to execute - sometimes more than once.

Hey Stephen, thanks for all your responses with my thousands of questions. :)

This thread won't be necessary anymore since I have to convert my XRaid to Flexraid per the other discussion. I made more space so now I'm going to be able to move all 5Tb of data over to external drives and slap all 6 drives in and do a one time configuration.

You are moving fast!

Hope the reset works well.

lol yah I'm tired of juggling between 2 NAS. I've convince myself that the NAS interface is not a critical part of the nas functionality. DSM is very nice, having OS6 would be extremely nice but I've settled to live with OS4. At the end of the day, having a RAID and multiple backups is for a peace of mind so I'm not going to do a hack job by keeping DSM or OS6 on this device and to drop a couple grand on a new NAS is out of the question.