I’m at the end of the pre-read. It’s taken about 20 hours. It also says that’s step 1 of 5. Can I ballpark the total time to 20hrs x 5 =100hrs total?

these are 17TB drives. 
Whew. 

Not all steps are of equal length so 100 hours will be pessimistic.    Basically there are 3 time-consuming steps with the pre-read and write phases taking about the same time, and the post-read being a bit slower.    However as you are seeing this is definitely not a fast process with the very large sizes now reached by modern drives  
 

This is probably why more people are not bothering to use the pre-clear process as it is not mandatory (unless you want to do an initial disk stress test).

Not mandatory anymore? Cool! Been a while since I made a new system. I can just assign the drives and it’ll be all good?

6 minutes ago, dkerlee said:

Not mandatory anymore? Cool! Been a while since I made a new system. I can just assign the drives and it’ll be all good?

Yes in principle.

 

if you just assign the drives as data drives to an array that is already parity protected then Unraid will carry out a ‘Clear’ operation on the drive to ensure parity remains valid before you can format and use the drive for storing data.  This will take about the same time as the 1st phase of pre-clear (about 20 hours in this case).  This ‘Clear’ step is skipped if the disk has been pre-cleared so you do at least get back some of the time the pre-clear would take.
 

The array will remain available for use while this ‘clear’ operation is in progress.   This was not true in Unraid v5 which was the original impetus behind the creation of the pre-clear script as that script could be run as a background task with the array online.   Nowadays the only real reason for using pre-clear is to act as a stress test of a drive before adding it to the array as it is much easier to handle a hardware failure of a drive before it is added to the array.   As well as being useful for older drives of unknown provenance/status this can help pick up ‘infant mortality’ on brand new drives.

Great. I was happy to cancel all that stuff. They’re formatting now. Any way I can see status of the format?

 

 

 

10 minutes ago, dkerlee said:

Great. I was happy to cancel all that stuff. They’re formatting now. Any way I can see status of the format?

not that I know of - but it should only take a few minutes so that is not normally a concern.   If it appears to be taking longer then try refreshing the ‘Main’ tab in case it is just a GUI update issue.