I’m upgrading my 4TB drives in the NAS to 12TB drives. “Let’s be clever,” I thought (foreshadowing), “and do two things in parallel!”

So installing the next drive into the disk array of my Unraid NAS, I let Unraid do these tasks:

1. Replace a 4TB disk in the array with a 12TB disk. This takes a while to rebuild the 4TB disk’s content (and zero-out the rest of the disk). Took more than 20 hours.
2. Preclear the next 12TB disk in parallel via USB so that Unraid has to do less work when I install it.

Does that speed things up, though?

No, not at all, because Preclearing a disk takes 3x as long: Around 21 hours for each step that checks every nook and cranny of the HDD.

Here’s the log for you:

####################################################################################################
#                              Unraid Server Preclear of disk D7H8MTAN                             #
#                            Cycle 1 of 1, partition start on sector 64.                           #
#                                                                                                  #
#   Step 1 of 5 - Pre-read verification:                  [21:02:43 @ 158 MB/s] SUCCESS            #
#   Step 2 of 5 - Zeroing the disk:                       [21:13:34 @ 157 MB/s] SUCCESS            #
#   Step 3 of 5 - Writing Unraid's Preclear signature:                          SUCCESS            #
#   Step 4 of 5 - Verifying Unraid's Preclear signature:                        SUCCESS            #
#   Step 5 of 5 - Post-Read verification:                 [21:02:35 @ 158 MB/s] SUCCESS            #
#                                                                                                  #
#                                                                                                  #
#                                                                                                  #
####################################################################################################
#       Cycle elapsed time: 63:18:59 | Total elapsed time: 63:19:00                                #
####################################################################################################

####################################################################################################
#   S.M.A.R.T. Status (device type: default)                                                       #
#                                                                                                  #
#   ATTRIBUTE                   INITIAL CYCLE 1 STATUS                                             #
#   Reallocated_Sector_Ct       0       0       -                                                  #
#   Power_On_Hours              0       63      Up 63                                              #
#   Temperature_Celsius         20      39      Up 19                                              #
#   Reallocated_Event_Count     0       0       -                                                  #
#   Current_Pending_Sector      0       0       -                                                  #
#   Offline_Uncorrectable       0       0       -                                                  #
#   UDMA_CRC_Error_Count        0       0       -                                                  #
#                                                                                                  #
#                                                                                                  #
####################################################################################################
#                                                                                                  #
####################################################################################################

Preclearing is supposed to be useful to make sure the new drive is ok.

But it’s annoying as hell to wait 3 days.

A few years ago, prior to Unraid v6, pre-clearing was great to prepare new drives for the array for quick replacements, because the array had to be stopped to accept a new drive back then. Nowadays, Unraid can incorporate a new drive while the array is running, though. That means you can use the array’s Docker container, VM’s, and NAS to put your files away while Unraid clears the disk once.

There is still a benefit of checking the whole disk before you install it. I won’t be doing that for the next drive, though.