EDIT 2
More commands and info.
I feel like an idiot forgetting to include this originally. This is SMART values for /dev/sda. I have 1 Current_Pending_Sector, and 0 Offline_Uncorrectable.
SMART Attributes Data Structure revision number: 16
Vendor Specific SMART Attributes with Thresholds:
ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE
1 Raw_Read_Error_Rate 0x002f 100 100 051 Pre-fail Always - 166
2 Throughput_Performance 0x0026 055 055 000 Old_age Always - 18345
3 Spin_Up_Time 0x0023 084 068 025 Pre-fail Always - 5078
4 Start_Stop_Count 0x0032 100 100 000 Old_age Always - 75
5 Reallocated_Sector_Ct 0x0033 252 252 010 Pre-fail Always - 0
7 Seek_Error_Rate 0x002e 252 252 051 Old_age Always - 0
8 Seek_Time_Performance 0x0024 252 252 015 Old_age Offline - 0
9 Power_On_Hours 0x0032 100 100 000 Old_age Always - 12224
10 Spin_Retry_Count 0x0032 252 252 051 Old_age Always - 0
11 Calibration_Retry_Count 0x0032 252 252 000 Old_age Always - 0
12 Power_Cycle_Count 0x0032 100 100 000 Old_age Always - 75
181 Program_Fail_Cnt_Total 0x0022 100 100 000 Old_age Always - 1646911
191 G-Sense_Error_Rate 0x0022 100 100 000 Old_age Always - 12
192 Power-Off_Retract_Count 0x0022 252 252 000 Old_age Always - 0
194 Temperature_Celsius 0x0002 064 059 000 Old_age Always - 36 (Min/Max 22/41)
195 Hardware_ECC_Recovered 0x003a 100 100 000 Old_age Always - 0
196 Reallocated_Event_Count 0x0032 252 252 000 Old_age Always - 0
197 Current_Pending_Sector 0x0032 100 100 000 Old_age Always - 1
198 Offline_Uncorrectable 0x0030 252 100 000 Old_age Offline - 0
199 UDMA_CRC_Error_Count 0x0036 200 200 000 Old_age Always - 0
200 Multi_Zone_Error_Rate 0x002a 100 100 000 Old_age Always - 30
223 Load_Retry_Count 0x0032 252 252 000 Old_age Always - 0
225 Load_Cycle_Count 0x0032 100 100 000 Old_age Always - 77
# mdadm -D /dev/md0
/dev/md0:
Version : 1.2
Creation Time : Thu May 5 06:30:21 2011
Raid Level : raid1
Array Size : 1953512383 (1863.01 GiB 2000.40 GB)
Used Dev Size : 1953512383 (1863.01 GiB 2000.40 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Update Time : Tue Jul 3 22:15:51 2012
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Name : server:0 (local to host server)
UUID : e7ebaefd:e05c9d6e:3b558391:9b131afb
Events : 67889
Number Major Minor RaidDevice State
2 8 1 0 active sync /dev/sda1
1 8 17 1 active sync /dev/sdb1
As per one of the answers: it would seem I did switch seek and skip for dd. I was using seek as that's what is used with the HOWTO. Using this command causes dd to hang:
# dd if=/dev/sda1 of=/dev/null bs=4096 count=1 skip=401594986
Using blocks around that one (..84, ..85, ..87, ..88) seems to work just fine, and using /dev/sdb1 with block 401594986 reads just fine as well (as expected as that disk passed SMART testing). Now, the question that I have is: When writing over this area to reassign the blocks, do I use /dev/sda1 or /dev/md0? I don't want to cause any issues with the RAID array by writing directly to one disk and not having the other disk update.